Twenty-five million Americans with borderline high blood pressure, the kind generally regarded as "high normal" and often left untreated, can sharply decrease their risk of death through a careful treatment program, federal health officials reported yesterday.

Dr. Robert I. Levy, director of the National Heart, Lung and Blood Institute, said a five-year study of 10,940 men and women has shown that persons with this mild hypertension, or high blood pressure, could cut their death rate by an average 20 percent by "vigorous" treatment designed to reduce their blood pressure.

In whites, the study showed, the annual death rate could be cut by 10 percent, compared with persons who get only "usual" medical care, and in blacks, who suffer more high blood pressure, the rate could be cut 22.4 percent.

Treatment, Dr. Levy said, may merely be diet to reduce excess weight or cut salt intake; or it may be other health measures -- exercise in many cases -- or, when none of this works, one of several drugs.

Mild or borderline blood pressure was defined as pressure between 140 over 90 and 160 over 95. The first number is the systolic pressure when the heart contracts and pumps blood; the second is the diastolic pressure when it relaxes.

Doctors for years have debated the value of treating hypertension in the range of 90 to 100 or 105 diastolic. Some have called this "high normal" and not worth treating. What many have been awaiting is a demonstration, such as this one, of the value of treating the mild hypertensives.

The same study, Dr. Levy added, has also shown that anyone with high blood pressure, even the highest, may hope to "live considerably longer" if adequately treated.

If everyone now under treatment for high blood pressure were given truly adequate care, he said, it might be possible to reduce premature death among all hypertensives by 17 percent, saving 120,000 lives yearly.

The adequate care is the kind given 5,485 men and women following for five years at 14 medical centers in the heart institute's Hypertension Detection and Follow-up Program, or HDFP.

The HDFP patients were compared with 5,455 who had similar blood pressures but got the "usual" care from their own doctors, hospitals or health centers. This ranged from no care at all, virtually, to the same intensive care received by the HDFP group. In general the care was "good" by past standards, at least, Levy said.

These in the HDFP group, on the other hand, were intensively studied and interviewed, and then various treatments were tried.

This often included many visits to find an effective way of life or in two cases in three, a drug. Some patients had to try several drugs, since some cause unpleasant reactions.

What was important was follow-up, Levy and participating doctors from several states said. Doctors, nurses, social workers and others listened sympathetically to patients' complaints about drugs or other treatment. They reminded their patients of appointments and the importance of returning for periodic checkups.

They emphasized the value of staying on treatment, even in the absence of any symptoms. The insidious thing about high blood pressure is that it is usually symptom-free until there is a stroke or heart attack or related event. The "event," when it happens, is often fatal.

The care the intensively treated patients got in the $17 million federally financed study was free. This could be a factor in its success, especially among minorities and others in doctor-short areas, Levy conceded. Anti-hypertension drugs can cost a patient from $125 to $200 a year.

But the main thing the study has shown, said Dr. Jeremiah Stamler of Northwestern University, is that "misconceptions" by doctors and patients, not poverty, have been the No. 1 obstacle to blood pressure control. And the main misconception, until yesterday, was the feeling that mildly elevated pressure is harmless.