You're in a hospital about to undergo knee replacement surgery and you're worried that the doctor might make a mistake and operate on the wrong knee.

Do you:

(A) Decide you're being silly, there's nothing you can do except trust the doctor to get it right.

(B) Tell everyone with whom you come in contact--orderlies, nurses, the anesthesiologist and, of course, the surgeon--which knee is the bad one.

(C) Write "NOT THIS SIDE" on your good knee.

The only wrong--or at least ill-advised--answer is likely to be A, according to patient advocates and experts who study medical errors. When it comes to avoiding or minimizing the chance that you or a loved one will be the victim of a medical mistake, the best advice is to--as former president Ronald Reagan put it--trust but verify.

Public concern about medical mistakes has intensified in the past two weeks, spurred by the publication of a bluntly worded report by the Institute of Medicine, a branch of the National Academy of Sciences.

An IOM panel concluded that medical errors, many of them preventable, kill an estimated 98,000 Americans annually. That number is more than the toll from breast cancer, traffic accidents or AIDS.

Last week President Clinton ordered more than 300 health plans that sell insurance to federal workers to strengthen programs to reduce errors. Several lawmakers have said they plan to introduce legislation designed to curtail errors when Congress returns in January.

While experts say that many errors are systemic in nature and lie in the way doctors, nurses and pharmacists practice, there are steps individual patients can take to reduce their chances of being hurt or even killed by mistakes.

Here are some suggestions:

* Become informed. Too many patients don't know what's wrong with them or what they're being treated for, consumer advocates say. Becoming informed about your medical problems and about treatments is more important than ever, said Arthur Levin, director of the New York-based Center for Medical Consumers and a member of the IOM panel.

"There's a real self-interest here in not being lazy," said Levin. "People really need to take the time to understand more about their medical issues. If you don't know why you're taking medicine, you're not going to recognize that you might be getting the wrong drug or the wrong dose."

* Ask questions. Do not assume that you are getting the right drug or the right dose simply because a doctor prescribed it for you or a nurse tells you it's time to take it.

If you're in the hospital, ask what you're taking. If you don't recognize the name of the medicine, ask why it's being given.

This is especially important for elderly patients, who tend to take more medications and whose drugs may negatively interact with each other.

If you're allergic to a drug, make sure you tell everyone with whom you come in contact, and see that this information is written clearly on your chart. Medication errors, such as giving patients drugs to which they have a documented allergy, are among the most common mistakes in hospital settings, the IOM found.

Ask similar questions about a procedure or treatment if you're told one has been scheduled and you don't know what it's for or why it's necessary. "Don't be afraid that you're going to offend the system," said Michael Millenson, of William Mercer, a Chicago benefits consulting firm, who has written extensively about medical quality.

* Monitor hand washing. Numerous studies have documented the low rate of hand washing, particularly by doctors. Half of all hospital-acquired infections are preventable, according to the Centers for Disease Control and Prevention. Latex gloves, which are ubiquitous in hospital settings, don't protect patients unless they are clean. Ask that anyone who touches you or a relative change their gloves or, better still, wash their hands.

* Check out your doctor. This is impossible if you're brought into an emergency room but not if you're having an elective procedure. A growing number of states have computerized databases that list information about doctors; these databases vary in quality and the amount of information provided but are worth checking out.

Avoid any doctor who does not have hospital privileges and wants to perform surgery in his or her office; there is virtually no oversight in a doctor's office. Think twice about accepting treatment from a doctor practicing outside his or her specialty; many have little experience and limited training.

* Acquire an advocate. Being in a hospital or having a procedure is stressful. That's why having an advocate--a family member or a friend who is not sick or vulnerable and is more likely to remember things and to ask relevant questions--makes sense.

"I don't think any physician or nurse would say they would go into a hospital and not have someone looking out for them," said Millenson.

A trusted advocate may be especially important for elderly patients who may be confused or unable to fend for themselves.

The role of an advocate can run the gamut of concerns about hospital care, such as making sure that a patient is getting the right food, enough painkillers, the appropriate drugs and proper treatments.

* Practice full disclosure. Don't shop around frequently for new doctors, or if you do, tell the doctors who treat you what medications you're taking, including herbal or over-the-counter products. Some errors occur when patients don't tell their doctors what drugs they're using.

Consider using one pharmacy, which is more likely to keep your records in a central location on a computer. A pharmacist who knows you or has access to your complete records is more likely to spot an error and may use software designed to detect harmful drug interactions or allergies.

* Don't accept illegible prescriptions. If you can't read a prescription, request a more legible copy. Some errors are the result of doctors' notoriously bad penmanship. A Texas jury recently awarded a $450,000 judgment to the family of a 42-year-old man who sued a cardiologist and a pharmacist. The man died after being given an unusually high dose of Plendil, a drug to treat high blood pressure. The cardiologist had written a prescription for Isordil, a drug to treat angina.