Thousands of Hispanic patients stream though the East Los Angeles practice of Dr. Hector Flores and his partners each year.

The older ones go to the family practice with arthritis and hypertension, the younger ones with diabetes and asthma.

What surprises Dr. Flores, however, is not how sick they are, it is how sick they are not. Overall, Dr. Flores said, his patients are much healthier than one would expect given their low levels of income and education, factors epidemiologists long have known are linked to poor health.

“You can predict in the African-American population, for example, a high infant mortality rate,” he said recently, “so we would think a [similarly] poor minority would have the same health outcomes.

“But they don’t. They’re not there,” he said, referring to outcomes among Hispanics.

Why Hispanics are not sicker — a phenomenon known to health experts as the Hispanic paradox — is puzzling to public-health experts, given the link between disadvantage and high disease and mortality rates.

In overall mortality rates and infant mortality rates, two standard measures of a population’s health, Hispanics’ numbers approach and sometimes surpass those of self-identified non-Hispanic whites.

In Los Angeles County in 2003, the age-adjusted mortality rate for Hispanics was 535 out of 100,000, 33% less than for non-Hispanic whites and 52% less than non-Hispanic blacks, according to the most recent data from the county’s Department of Public Health.

Nationally that year, Hispanics’ mortality rate was 621, 25% less than whites’ and 43% less than blacks’, according to National Vital Statistics Reports, published by the Centers for Disease Control and Prevention.

Hispanics’ infant mortality rates reflect a similar pattern. In Los Angeles, the rate was 5.2 out of 100,000 in 2003, 16% higher than whites’ and 57% less than blacks’. The national rate was 5.7, about the same as non-Hispanic whites’ and 58% less than blacks’.

“It violates one of the most predictable patterns we see in most areas of the world and for most diseases,” the chief epidemiologist for the Public Health Department, Dr. Paul Simon, said. “The question is, ‘What is the Latino population doing right?'”

The reasons for the paradox are a matter of some debate. Some scholars attribute it to immigration, which may draw selectively from the ranks of the hale and hardy. Another possibility is that many immigrants return to their home countries when seriously ill, skewing mortality statistics in this country.

But increasingly, researchers are suggesting that such factors as diet, lifestyle choices, and strong social support networks are the key to Hispanics’ better-than-expected health.

“They’re not taking some secret Aztec herb they didn’t tell you about,” David Hayes-Bautista said. Dr. Hayes-Bautista is an early observer of the Hispanic paradox who directs the University of California, Los Angeles, Medical School’s Center for the Study of Latino Health and Culture.

It is worth figuring out what is making the difference, he added, because “we could all be better off for it.”

As the immigration debate heats up and the cost of health care soars, the phenomenon is attracting attention from social scientists and public-health officials. It was first noticed, though, in the 1970s and 1980s by researchers looking at infant and overall mortality rates in Texas and California.

Scholars tended to view the findings as wrong or anomalous, assuming that Hispanics’ relatively disadvantaged socioeconomic status put their health status more on par with blacks’.

As data accumulated, covering broader swaths of the country and longer periods, skepticism turned to curiosity.

Kyriakos Markides, who in 1986 coined the term “Hispanic epidemiological paradox,” described scholars’ shift in recent years as remarkable.

“Nobody talked about it then,” Mr. Markides, a professor of socio-medical sciences at the University of Texas Medical Branch, said, speaking of a generation ago. “People just ignored the data or assumed that disadvantaged populations have high mortality. Now, it’s the leading theme in the health of the Hispanic population in the United States.”

The paradox remains debated, in part because its first-blush message — that Hispanics are less ill than expected — might lead people to believe that the group does not need scarce health-care dollars.

Dr. Hayes-Bautista recalled an incident in the 1990s when a colleague whom he had just briefed on the paradox asked him to keep the data to himself, saying she feared services for Hispanics could be cut.

“I guess it’s the triumph of — I hate to say — of ideology over data,” he said.

Studies have indicated, however, that Hispanics drink less alcohol and smoke less than whites, although their healthful behaviors appear to wane with greater acculturation in America.

Several studies have found that the children and grand-children of foreignborn Hispanics tend to smoke, drink, and use illegal drugs more than their parents and grandparents. And some research suggests that they are less likely to breast-feed and to stick to the healthier diets of their forebears.

“You can see it in obesity rates; they’re very high in Mexican children,” an associate professor of medicine and public health at UCLA who has written about the paradox, Dr.Leo Morales, said.

“That portends high disease rates for cardiovascular disease, diabetes, all the obesity-related complications.”

An assistant professor at Columbia University’s Mailman School of Public Health, Ana Abraido-Lanza, cited a greater availability of fast food in this country and social factors, including a more relaxed attitude toward drinking.

Another possible culprit is stress.

A 1999 study published in the journal Social Forces found that Puerto Rican women who had recently moved to the mainland reported fewer stressful “life events” than mainland-born peers, such as being physically abused or being close to someone with a serious drug or alcohol problem.