As far back as she can remember, 42-year-old Dorothy Dunbar of Arlington, a one-time jogger and a champion swimmer, would occasionally experience pressure in her chest or episodes in which her heart would flutter uncontrollably or skip a beat here and there. But she never felt alarmed by the symptoms, often attributing them to stress or too much exercise. "I would just stop what I was doing until the discomfort would go away," she says.

Eight years ago, however, when Dunbar failed a cardiac stress test because of tightness in her chest and severe palpitations, she realized the significance of symptoms she once dismissed as being "merely bothersome."

Dunbar discovered she has mitral valve prolapse, a structural abnormality that interferes with uniform blood flow through one of the valves of the heart.

Frightened and confused about how ill she was, Dunbar says her mind raced with thoughts like, "How much longer do I have to live?" and "How do I make whatever time I have left more meaningful?"

Dunbar's disease isn't lethal, but the terror she felt is common among those who discover they have heart disease, says Katherine Shear, a psychiatrist and director of the anxiety disorders clinic at New York Hospital-Cornell Medical Center.

"Many people get worried and preoccupied about the condition of their heart as a result of a physician telling them that something is wrong with it," she says. Shear urges physicians to paint a realistic picture of this syndrome.

Once Dunbar's physician explained that her condition was not terminal, but rather was relatively benign, Dunbar began to relax. "Accepting what you have and living with it is the important thing," she says.

Physicans have seen mitral valve prolapse with greater frequency in recent years. Dr. J. Michael Criley, chief of cardiology at Harbor-UCLA Medical Center in Torrance, Calif., and the man who named the condition in 1962, attributes this phenomenon to greater physician awareness and the introduction of sophisticated diagnostic equipment.

Two decades later, the Framingham Study, a National Heart, Lung and Blood Institute project that is following a group of healthy people in Framingham, Mass., to determine what diseases they develop during their life, estimated that 5 to 10 percent of all Americans have mitral valve prolapse, and in younger, tall, thin women, as many as one out of four may have it.

These findings have led Criley and other researchers to question whether mitral value prolapse is a disease or just a variation of normal.

Many patients with mitral valve prolapse are in excellent shape. Dr. H.V. Palmer, former team physician for the Los Angeles Clippers basketball team, sees the condition in 50 percent of the tall forwards and centers on the team. "They've come up the ranks with the condition and have had no problems. It would be ridiculous to worry about it now," he says. Palmer estimates that at least half of all National Basketball Association players have the condition.

"Mitral valve prolapse can't be that dangerous if we're seeing it in 90-year-olds," says cardiologist William Castelli, director of the Framingham Study. The study will follow the prolapse patients for four to six years, and Castelli says, "I suspect the participants will do just fine.

"Mitral valve prolapse is like a squeaky door. Even though a door might squeak and make other noises, it will still open and close. The same is true of mitral valve prolapse," he says. "Even though the mitral valve makes noise in most patients, it still functions well for the most part."

Nearly 100 years ago, the somewhat musical sounds of mitral valve prolapse were first described. The heart's valves separate its four chambers and regulate the coordinated movement of blood. The mitral valve consists of two flaps that separate the left upper chamber or atrium from the left lower chamber or ventricle. When the valve opens, blood flows from the atrium to the ventricle.

In some people, one or both of the valve's flaps prolapse, or balloon up like a sail toward the atrium, when the ventricle contracts to pump blood out of the heart through the aortic valve. As the valve prolapses, it makes a clicking sound. Because the valve flaps do not close smoothly -- they resemble a double door that swings back and forth until it comes to rest in line -- they sometimes allow a small amount of blood to leak back into the atrium. The leaking blood creates a whooping sound that may crescendo at the end of a contraction. This sound is called a murmur.

Most patients with mitral valve prolapse have only a click. A small number have a click and a murmur and still others will have neither a click nor a murmur and are totally symptom free.

Besides the heart's unusual sounds, patients may complain of palpitations, fatigue, shortness of breath, irregular heart beats, and chest pain that is occasionally mistaken for angina. These symptoms are seldom related to exertion and generally last longer than anginal pain -- the chest pain caused by insufficient blood flow through narrowed coronary arteries -- a condition that may lead to a heart attack.

"However, these symptoms can represent mitral valve prolapse as well as a number of other diseases or even no disease," says John S. Gottdiener, a cardiologist at the Washington Veterans Administration Hospital.

The cause of the condition is not clear, but since it can be inherited and since it has been associated with certain skeletal or connective tissue disorders such as Marfan's Syndrome -- in which sufferers are tall and lanky and have stooping shoulders -- a genetic basis has been suggested. Lincoln is thought to have suffered from Marfan's, and perhaps, it has been speculated, from mitral valve prolapse as well.

Physicians generally treat this condition by reassuring patients that they will lead long, productive, problem-free lives.

Sometimes, antibiotics are prescribed during surgery or dental treatment to prevent infection of the valve, a condition known as endocarditis, but this practice has been controversial.

Cardiologists Criley believes it's a good idea to take antibiotics as a precaution. The treatment also is recommended by the Committee on Prevention of Rheumatic Fever and Bacterial Endocarditis of the American Heart Association. The committee believes that with any dental treatment or surgical procedure involving the upper respiratory, genital and urinary or lower gastrointestinal tract, bacteria are likely to enter the bloodstream and become attached to the abnormal valves, causing bacterial endocarditis, an inflammation of the membrane lining the heart.

A patient with endocarditis could spend six to seven weeks in a hospital, and there is a chance that blood clots could form, break off and clog the blood flow to major organs. "It's a horrible illness to have," says Criley. "It could be life-threatening." He estimates that 5 percent of those who have mitral valve prolapse will get endocarditis.

Cardiologist Gottdiener, however, doesn't prescribe prophylactic antibiotics for patients with just a click, but does for patients with both a click and a murmur.

"How to treat a patient with mitral valve prolapse is one of the major controversies in medicine," says Gottdiener. "It's hard to balance reassuring the patient and sometimes scaring them at the same time."

Despite the controversy over how to manage it, most people with the condition simply continue to live normal lives.

Dorothy Dunbar, director of swimming pools for Arlington County, still swims. She just doesn't sprint unless she's thoroughly warmed up. "Having mitral valve prolapse hasn't slowed me down any," says Dunbar. "I don't look at it as a do-or-die situation. I look at it as something you try and understand and go on and do the best you can despite it."