Women who have heart attacks in middle age are more likely to die than men for reasons that have nothing to do with sex discrimination, quality of treatment, type of medical insurance or complicating illnesses, according to a new study.

The differences are quite large in the youngest age groups. Under the age of 50, for example, about 6 percent of women with heart attacks die in the hospital, compared with about 3 percent of men. The differences slowly decrease until they disappear at about age 75.

The findings do not suggest that middle-age women are more likely to have heart attacks than men. In fact, women are much less likely to have them. However, when they do, they appear to be at markedly higher risk of dying.

"It tells us there may be a special group of women -- younger women -- whose heart disease has special characteristics very different from men," said Viola Vaccarino, a physician and epidemiologist at Yale University School of Medicine who headed the study.

Over the years, various studies hinted at a "sex-specific" risk of dying after a heart attack, and many people assumed the difference (if it existed) arose from behavior. Among the explanations was that doctors provided less aggressive medical care to women and that women often went to the hospital later because their symptoms were less typical than men's.

The new study, the largest of its kind, confirms the mortality difference is real. It also strongly suggests that behavior explains very little of it.

Vaccarino, and colleagues at the University of Washington and the University of California in San Francisco, looked at the experience of about 385,000 people in the National Registry of Myocardial Infarction 2. Opened in 1994, the registry contains information on people admitted with heart attacks to more than 1,600 American hospitals. In this study, about 156,000 of the patients were women and 229,000 were men.

Death rates for people under 50 were 3 percent for men and 6 percent for women. For ages 50 to 54, they were 4 percent for men and 7 percent for women. By ages 75 to 79, however, the differences had narrowed considerably, standing at 18 percent for men and 19 percent for women. In the 80-to-84 age group, the risk of death was roughly 22 percent for each sex.

Overall, for every five-year decrease in age (starting at age 75), the odds of a woman dying in the hospital after a heart attack increased 11 percent, compared with men.

The researchers used various statistical methods to adjust for differences between the male and female patients. There were many. For instance, women were less likely to have revealing EKG tracings and were more likely to have no chest pain, two conditions that make it difficult for a physician to make a quick diagnosis. Unfortunately, women also were more likely than men to have some problems -- notably congestive heart failure -- that increase the chance of death.

Women were slightly less likely to get clot-dissolving drugs, an important and sometimes life-saving treatment. They were more likely to have diabetes and less likely to have commercial health insurance. Vaccarino and her colleagues assumed they would find one or two variables that contributed most of the difference in risk of death between women and men.

Surprisingly, they did not. Only about one-third of the difference in death rates between the sexes disappeared when they, effectively, made the two groups identical.

"We took into account everything we could," she said, "but the greater portion of the differences between younger men and younger women is still unexplained."

What does explain the difference? Nobody knows. Relatively young women -- especially those who have not reached menopause -- have not been studied much by heart researchers because heart disease is much less common in them than in men.

The rate of "new or recurrent" heart attack between age 65 and 74 is 26 per 1,000 white men, but only 8 per 1,000 white women. For black men of the same age group, it is 16 per 1,000, and for black women, 13 per 1,000. The sex differences are even more dramatic at younger ages.

Estrogen, the hormone of femaleness, has myriad beneficial effects on the cardiovascular system. It is believed to account for most of these differences in rates of heart disease.

However, it is possible that a small minority of women is innately resistant to estrogen's beneficial effects, and that they are the ones having heart attacks at a young age. That condition (or other, still unidentified ones) may make them less responsive to treatments, such as aspirin, clot dissolvers and beta-blocker drugs, that help men.

It is also possible that aspects of behavior not measured in the study may explain some of the sex differences found.

Vaccarino noted that a larger percentage of women than men were black and on Medicaid, the government health insurance plan for poor and disabled people. People of lower socioeconomic status may be more likely to be depressed, or lack social support.

Both of those, she said, are known to increase a person's risk of dying after a heart attack.

CAPTION: WOMEN'S RISK (This chart was not available)