Patients who have mild or moderate symptoms of heart disease could safely skip or postpone costly coronary bypass surgery, a six-year study of heart patients shows.

Some 25,000 coronary bypass operations could be eliminated each year, enabling the nation's hospitals to save at least half a billion dollars without endangering patients, according to the study commissioned by the National Institutes of Health.

Coronary bypasses--operations to graft new blood vessels onto diseased hearts, bypassing plugged coronary arteries--in the last 15 years have become one of medicine's most common procedures. Nearly 200,000 such operations will be done this year at a cost of $15,000 to $40,000 each, including a surgeon's fee of $2,500 to $5,000..

A group of doctors compared the results of surgery with the results of nonsurgical treatment of patients with mild or moderate symptoms of heart disease. The results were virtually the same, Dr. Thomas Killip of Henry Ford Hospital in Detroit reported at a news conference yesterday at NIH.

Speaking for physicians at 11 medical centers who made the study for NIH's National Heart, Lung and Blood Institute, Killip said:

* Coronary bypass surgery is "clearly indicated" for many patients with serious disease, specifically those with nearly complete blockage of the left main coronary artery--one of the heart's most important vessels--or those with "unacceptable" angina or pain and severely restricted lives. In these two groups, bypass operations can lengthen life and improve its quality.

* Patients with mild angina, however, or angina-free patients with less severe blood vessel blockage, have virtually the same survival rates as persons who don't have surgery.

Many patients with mild angina are having surgery. Killip said it is true that surgery usually provides the patient with mild angina more relief from pain than does other medical treatment.

But in most cases, he added, the affected blood vessels gradually get worse again and the patient may need a second operation, with greater risk and less chance of a good result.

So the "best strategy" for the mildly affected patient, he said, is to "stay with medical rather than surgical treatment" unless or until the condition gets worse and surgery is clearly indicated. Killip said it is a matter "of having your one good operation at the best possible time."

The $24 million National Heart Institute study was conducted at 10 American centers: the University of Alabama, Albany Medical College, Boston University, the Marshfield (Wis.) Clinic, Massachusetts General Hospital, Milwaukee Veterans Hospital, New York University, St. Louis University, Stanford University and Yale University and at the Montreal Heart Institute.

Doctors randomly divided 780 volunteer patients with mild heart disease or heart symptoms into two groups. Ninety percent of the patients were men. Their average age was 51, and none was over 65.

Half had operations. Half had medical treatment consisting of drugs and advice to start exercising sensibly and avoid risks like smoking, overeating and consuming too much fat in their diets.

After six years, 92 percent of the surgical patients and 90 percent of the medical patients are still alive.

Among one sub-group--those whose hearts were failing to pump enough blood, according to X-ray studies--the results of surgery were slightly better: 92 percent of these surgical patients were alive after five years, compared with 84 percent of similarly afflicted medical patients.