"I have been taking care of cancer patients for a long time," says Dr. Vincent DeVita Jr., director of the National Cancer Institute.

"I have never taken care of a doctor who didn't get a second opinion.

"I have never taken care of a doctor who didn't have his microscopic slides read twice -- by different pathologists -- just to make sure that he had cancer.

"I think there is a message in that."

When should you seek a second opinion or a consultation with another doctor about a diagnosis of cancer or any other serious disease?

About a recommendation for surgery or any other treatment that can sometimes hurt rather than help?

Or when you think you're not getting well despite your doctor's efforts?

The answer:

"Whenever you get worried, whenever you get uncomfortable enough to need to ask," says Dr. Harry Kuykendall, an Alexandria family doctor.

"I say that to my patients with some frequency," adds this plain-speaking physician who currently leads the Medical Society of Virginia. "If you come to me and I evaluate you and give you my opinion, and I detect some lack of conviction and I can't convince you, I often suggest that you ask someone else."

Yet getting "another opinion" can be one of the most difficult problems for patients to handle.

We may feel uneasy about a doctor's recommendations. We may think a drug we're taking is doing more harm than good. We may not feel sure our doctor is right. But the doctor is still "the doctor," the possessor of knowledge we do not share, and no matter how forthright we are in our other relationships, we often have trouble telling a doctor we want another opinion.

"Patients are timid -- too timid," says Dr. Marcia Angell, deputy editor of The New England Journal of Medicine.

"I think people have a tough time asking for a consultation," Dr. Michael Newman, a Washington internist, observes. "People worry about hurting their doctor's feelings or interrupting the continuity of the relationship. Or they're not quite sure how to proceed."

One way, sometimes necessary, is to say nothing and just go to another doctor, get that doctor's opinion, then either stay with your first doctor or not, as you think best.

A better way -- one far more likely to give you and your doctor the advantage of a true meeting of medical minds -- is to say to your doctor something like:

"I appreciate your help and your opinion, but I think I need another opinion to be comfortable."

Or -- often the case -- "my husband/wife/son/daughter/or significant other wants me to get another view."

This should not only get you the consultation you want, but may also help get you reimbursed by a health insuror, since there has now been a formal "referral."

Many people refer themselves, especially in an area like Washington with what seems like a dozen educated men and women, half of them amateur doctors, per square foot. In fact, an official of the American Academy of Family Physicians once said, "That's the biggest problem today in American medicine. A young couple gets married. She gets pregnant and takes herself to an obstetrician, who won't look after the baby. So they go to a pediatrician.

"Then the husband needs a doctor and goes to an internist. So their records, history and any real knowledge of their family problems are scattered around several doctors. Or lost. Or unknown. This is the way it's been since the big surge of specialization after World War II. We're trying to get it back together again."

Self-referral to specialist after specialist can be costly. The more visits, the more tests, drugs and fees and the more specialized the specialist, the higher the price.

It can also be dangerous. "A lot of people are not happy with what they hear from a doctor," Newman says. "So they get three, four, five opinions, waiting for someone to say, 'Just do this and the problem will go away.' They just keep hoping someone will tell them what they want to hear."

Eventually someone may.

And the diagnosis -- and treatment -- often depend on the expert. A woman with a severe backache went to an orthopedist. His considerable efforts failed to find a cause amidst her bones and her nerves. Later her primary-care doctor found she and her husband had been having sexual problems; her difficulty in having orgasms had caused pelvic congestion.

Many doctors say: After your consultation with a specialist, go back and discuss the outcome with your own doctor, who should be a generalist or primary care physician. That usually means a family practitioner or general-practice internist, a specialist in adult medicine.

At the Mayo Clinic, you are generally first seen and examined by an internist, who becomes your doctor. Then he or she refers you to whatever consultants seem advisable -- a surgeon, an orthopedist, a cardiologist, whatever. They then talk over the results. If an operation or any other treatment is recommended, "your doctor" again sees you and may sometimes recommend against a consultant's advice, on the basis of your overall situation.

"In my judgment," says Alexandria's Dr. Kuykendall, "you should not go to a specialist until you've seen someone like me. I'm a generalist, a board certified family physician, and theoretically you should be able to bring me anything you've got. If it's simple, I'll fix it. If it's more complicated than I can handle and does not respond to some standard steps, I'll refer you to an expert in that field.

"It's then my responsibility to refer you to someone whom I personally know is as capable as possible. There are doctors and there are doctors, and if a patient has to see an expert it should be the best expert I can come up with in all respects."

For your sake, your consultant should also be sturdily independent. If the expert turns out to be your doctor's best friend, it may be no "second opinion" at all.