That's funny, people say to Dr. Leslie Dornfeld, you don't look obese.

Dornfeld, who is 5-feet-9 and weighs about 150 pounds, insists on it. "I have obesity," is the way he puts it.

He ought to know. Obesity is the specialty of this former nephrologist, former hypertension specialist and current faculty member from the UCLA Medical Center. And moreover, he shows a slide of himself as a Marine -- 30 pounds heavier -- and concedes "That's not the worst. It's been a lot higher."

"I think," Dr. Dornfeld said recently to a group of staff physicians at Fairfax Hospital, "the time has come to recognize, as we did with hypertension a number of years ago, that obesity is a disease. And we have abrogated the treatment of this disease to a lot of lay people.

"It is a disease and it is a killer," he said, quoting statistics relating weight to heart disease, hypertension, gallbladder disease, and diabetes, for starters.

But Dr. Dornfeld is doing more than lecturing.

He is setting up a massive, nationwide computerized clinical study of tens of thousands of obese Americans as they are being treated in satellite obesity clinics. He was at Fairfax Hospital to conclude plans for its clinic, which is now ready to start screening volunteers.

The way Dr. Dornfeld sees it is this: "Obesity is a chronic disease just like diabetes or hypertension. People who have it will have it as long as they live. No matter what their weight is. It's a matter of their being controlled.

"If I'm a hypertensive on a blood-pressure medicine that keeps my blood pressure normal, I'm still a hypertensive.

"If I'm a diabetic on insulin which keeps my blood sugar normal, I'm still a diabetic."

Furthermore, believes Dornfeld, "That's probably the reason that as soon as the obese person has gotten his weight down, and you take him off the therapy -- whether it's a pill or a shot or behavior modification, whatever it is, and almost anything works for a little while -- he goes off on his own and thinks he can do it.

"Well, it's just as though a diabetic stopped taking his insulin, or a hypertensive went off his medication. It goes right back up. Whether it's blood pressure, blood sugar -- or weight."

The clinics are designed to help Dornfeld and his research colleagues prove his thesis, and, he hopes, find a way to conquer the killer, weight.

Clinics may be set up in hospitals or medical schools, private offices or health-maintenance organizations, commercial or public service, but all will have a major common element: Patients will be put on a modified fast.

The program's dietary supplement (Optifast) is, it is important to note, different in critical and significant ways from the liquid-protein supplements which have been marketed commercially and which have been implicated in heart arrhythmias and some deaths.

Optifast was developed at Case Western Reserve University Medical School, in Cleveland, Ohio. It has a record already of about 400,000 patient weeks with no adverse effects -- and a weight loss approaching 2 million pounds. The success of this program in weight reduction alone is remarkable.

Studies show that only about 5 percent of patients classified as "massively obese" can lose significantly at all and most of the lost weight is soon regained. Even with Optifast, concedes Dr. Dornfeld, if the patients "drop out of sight" after they've lost weight, "our record (of patients regaining lost poundage) is as bad as anyone's."

The secret is to maintain the contact between patient and program. Even the thin patient must remember he "has obesity."

Optifast -- the name is probably going to be changed -- is a meticulously concocted brew, made of highest-quality protein (as the commercial preparations were not), with carbohydrates, minerals, salts, vitamins, and trace elements carefully controlled and adjusted to specific biochemical needs. The formulation can and will be changed according to needs of research or patient.

Dornfeld regards the supplement as "a major research tool," permitting the most precise measurements and knowledge about relationships between what is eaten in what amounts and how various organs -- and psychic wellbeing -- are affected.

"We've known for a long time there are definite abnormalities in metabolism in obesity," he said. "The question is: How do they come about, and are they caused by obesity . . . or do they cause it?"

The Fairfax research will, among other things, investigate the relationship to obesity of endorphins and enkephalins: the newly discovered and still little-understood brain chemicals which act as the body's own pain killers and "high" producers.

Blood taken from patients will be subjected to the most sophisticated monitoring equipment, including radioactive assays which can be done at UCLA and at the National Institutes of Health. (Patients, of course, will simply provide the blood, which will be sent for testing to various points, depending on what is being tested as the research proceeds.)

Meanwhile, the patients themselves will be losing weight at the rate of about 3 to 5 pounds a week. They will be participating in various behavior modification or psychotherapy sessions, in groups or as individuals, and learning to understand the most crucial thing of all: the nature of their chronic illness, manifested by, as Dr. Dornfeld puts it, "an abnormal drive to eating which they cannot control." But which can be controlled with the help of trained professionals, peers, family support, but almost never alone, at least not for long.

And eventually, with the big guns of modern medicine and biochemistry trained on fat as never before, the disease of obesity may be at last identified and even vanquished.