IMAGINE THAT YOU KNOW you have a 50-50 chance of coming down in 20 years with an incurable brain disease that will make you insane, then kill you. Supgose a medical test were available now that would tell you with absolute certainty what your fate would be. Would you elect to take the test? Would you wish that the test be administered to your children?

These are not hypothetical questions. Technological advances are allowing us to make time-bomb predictions just like this. Through the use of available tests, patients with seemingly everday physical or emotional problems may be revealed to be on the road, years from now, to a full-blown, devastating case of multiple sclerosis, manic depression, Alzheimer's disease or schizophrenia. And I've come to think this new ability to peer into the futue places physicians and patients in an unprecedented position: in certain instances, the less each one of them knows, the better off they may be. Delving into the future at the cost of ruining what good years a person-at-risk may still have before him might be a Faustian bargain.

For example, not long ago I encountered an old enemy hidden within the brain of a 43- year-old man who came to my office to discuss a problem with his memory. His wife accompanied him and sat there tense and frightened while I completed my examination.

The man was right. His memory was failing and so, too, was his judgment, his temper, his mood and, little by little, his self-respect. But it wasn't until I saw a brief, sudden, jarring movement of his body -- almost as if the building had been rocked by a small earthquake -- that I knew what was wrong.

While still a resident, I encountered my first patient with Huntington's chorea. That man, too, had presented himself because he was dimly aware that the was losing his mind. At first there had been temper tantrums. They were followed by memory problems, jerking movement, insanity and then finally, mercifully, death.

My patient was also in an early stage in the illness and, to tell the truth, that made things even harder. Nothing is more difficult for a doctor than to tell a person he's afflicted with an illness for which there's no cure. The task is even more onerous when the illness destroys not only a person's body but his sanity as well.

Huntington's chorea -- which afflicted folksinger Woody Guthrie -- usually first shows up in middle age. At 43 my patient is just about the average age. It is caused by a single defective version of a gene lying on chromosome four. at would be a good thing.

Several weeks ago at the Americience meeting in Los Angeles, a researcher, Dr. Michael Phelps, showed how bioloington's chorea. He employed a PET scanner, which shows in motion-picture form the brain actually functioning, utilizing nutrients, performing chemical processes.

Phelps' main point was the PET scan can now show changes within the brain of a patient years before the illness manifests itself. "In 100 percent of the cases studied with PET," Phelps said, "we have pinpointed the altered chemical processes in the brains of the patients with symptoms even before it's detectable with other imaging procedures."

In the past six years, 50 percent of those patients with positive PET scans but no signs of the disease, have gone on to develop Huntington's chorea. Within the next five years, depending on developments in the other 50 percent of the patients, neuroscientists will know for certain whether or not PET scans can provide a predictive test for Huntington's chorea.

For the sake of making a point, let's concede that the PET scanner is every bit as reliable as Phelps suggests. This translates into scenarios that I'm not sure I want to have any part of:

"I am sorry to inform you that my examination confirms Huntington's chorea. About 50 percent of your children are at risk for later coming down with illness. If you wish, there is an available test called a PET scan which can tell us for certain which of your children will get the disease."

Or:

"Your PET scan is typical of a patient with schizophrenia."

None of these would exactly gladden your heart on a Monday morning.

I want to emphasize that we're not talking about some science-fiction fantasy. Not only will the test soon be available to any physician in the country. It's likely that other diseases, too, will soon be diagnosed ahead of time by means of PET scans or other tests. As a result, physicians will be in the position of being able to tell people more than they may wish to know or, in many instances, could be expected to handle.

This struggle for ignorance can be expected to create a particularly intense conflict within those physicians for whom the need to cure, learn more about, or at least understand and "control" illnesses provides their greatest intellectual and emotional satisfaction.

I don't mean to imply that this attitude is restricted to the medical profession, however. Indeed, our society at large is committed to the concept that "knowledge is power," that we are better off knowing than not knowing. We hate uncertainties, vagueness, having to "wait and see."

But in our urgency to know as much as we can about our present and future health prospects, we may find ourselves facing situations wherein knowledge isn't helpful at all and may, under certain circumstances, be positively harmful.

For instance, if my mother or father suffered from manic depression and perhaps died of suicide, would I care to have myself hooked up to a device capable of telling me whether or not my scan pattern is "similar to" or "suggestive of" a person afflicted with a major mood disorder?

Speaking personally, almost certainly I would not. But do I have the right to decide about such a test for one of my patients, believing as I do, that even to inform the patient of the existence of such a test is to create a conundrum that some individuals may be unprepared to deal with? In some individuals, such information might itself might lead to a sense of hopelessness -- even suicide.

From a legal point of view, patients have a right to know everything the doctor knows about their condition. But just how aggressive should the physician be in his diagnostic endeavors? Although I have a clear legal obligation to "fully inform" my patient about all aspects of the present illness and how it may affect him or her, do I have an obligation to tell anything more? After all, I'm not being consulted as a seer or a prophet. It seems reasonable that I should confine my interventions to the illness at hand.

What about extending my efforts to other members of the patient's family who haven't sought my help directly? They may justifiably resent my intrusion into their lives. For that matter, they do not suffer right now from an illness by any traditional definition of disease. This issue is far less easily resolved. For one thing, I am not convinced that even parents have the right to impose this kind of time-bomb information on their children.

For that matter, if my responsibility were to extend not just to my patient, but to my patient's children, where does this end? Public-health measures -- quarantine, vaccination programs -- are only some of the areas in which the physician administers to a wider group than simply the individual who seeks his services. It will not be very long, I believe, before health programs will urge a similar widening of the physician's obligation to come up with extremely early diagnoses of diseases that ultimately will require long- term treatment or even institutional care.

Alzheimer's, schizophrenia, manic-depressive psychosis, obsessive-compulsive disorders -- these are presently the principle candidates for predictive testing. But there will undoubtedly be more in the future. Notice, incidentally, that all of these disorders mentioned affect behavior. This, in itself, creates an additional dilemma.

Learning that one may later suffer from depression or senility is a far different matter than learning about the likelihood of arthritis (another medical condition for which predictive tests based on genetics are in the offing). Throughout our lifetime we have been taught to believe that "brain" and "body" means something totally different than "mind" and "soul." To speak of biological illnesses causing aberrant behavior is to introduce, many people believe, limitations on our free will that raises serious questions about our humanity -- are we any more than just the sum total of our bodies flaws? New techniques such as PET scanners are forcing such reevaluations.

Behavior is brain-based. By studying the brain, light can be shed on a host of behaviors. A dyslexia-caused reading problem, for instance, can be detected by studying a child's brain with a computer-enchanced electroencephalogram. Most intriguing is that in many instances these abnormalities can be detected even before the child is old enough to read.

While the early diagnosis of some disorders may be helpful and facilitate early intervention, many other disorders, including dyslexia, have so far frustrated the best attempts at treatment approaches that can be confidently anticipated to do any good.

Thus, although the issue is perhaps less severe (a reading disability versus a fatal illness), the conundrum is the same: something is diagnosed that will not have much affect on the person for years to come, and in the meantime, little can be done about it except worry.

Such dilemmas are different than any faced in the past simply because technology, rather than providing a solution as it has in the past (better drugs to better treat medical illnesses, refinements in surgical techniques and so on), has itself become the problem.

As a result, future technological advances and refinements can be counted upon to intensify the dilemma of who should know what and at what time. What tests should be ordered? By whom? Should tests that will only predict the apearance of the disease in the future be employed at all?

A particularly thorny problem involves new and innovative methods of testing intelligence.

On the way are intelligence tests which will get around the objections of the past (social and cultural disparities) but are likely to create problems of their own.

It's been found that, in general, the "intelligent brain" processes information quicker and more efficiently than average. It doesn't matter whether that "information" consists of flashing lights, beep tones or a story read aloud. By measuring the speed of nerve conduction within the brain, an estimate can be made of a person's general intelligence which is at least as reliable as the IQ tests presently in use.

But if the experience of the recent past regarding "IQ tests" and "tracking" programs in the schools is any indication of future direction, bitter conflicts surrounding this new technology can almost be guaranteed. Who should order the test? The schools? The doctor? And what kind of educational program should be provided for a child who performs below average?

Whatever the answers to such questions may be, I don't believe they are likely to be discovered via the development of newer and more sophisticated technology. The technology, by possibly making distinctions about intelligence more reliable, will quite to the contrary compound the problem.

In the meantime I'm left with the dilemma of what to do with patients such as mine with Huntington's chorea. There are certainly good and reasonable arguments that can be put forth in the cause of testing his children.

In the wings, only a few years hence, are fiances who reasonably might not wish to have a baby with an individual who may end his or her days demented and insane, and pass his flawed gene on to his offspring.

Enter now the insurance companies who undoubtedly could make good use of information regarding a prospective insuree's chances of dying at an early age through an incurable disease. "Since your father died of this disease, as a condition for writing your policy, you must submit to a PET scan."

Not only may illnesses soon be diagnosable across generations, but these tests may reveal aspects of a person's emotional and intellectual life that he or she may have never suspected and might wish never to be informed about.

In regard to my patient with Huntington's chorea, his three children can be flown out to Los Angeles, and, thanks to the wonders of modern technology, hooked up to a PET scanner. Only one thing is needed before that can happen, however. Somebody first has to tell my patient about the test and what it may portend.

Want to be left in on a secret? That person isn't going to be me.