After a five-year study covering more than 10 percent of the U.S. population, the National Cancer Institute has put together the most complete picture ever of how cancer affects Americans.

The study, completed this week, documents a major mystery: Hispanics develop at least one-third fewer cancers than other Americans do.

No one knows why, though there are several theories. Most involve special life styles, including diet.

There may also be genetic factors, since a lower rate of this dread disease is common to Puerto Ricans and Mexican-Americans, and a similar situation is reported in Latin America.

Doctors treating Hispanics have long been aware of the phenomenon. But it is documented for the first time in the NCI study of cancer incidence and mortality from 1973 through 1977 in 10 states or metropolitan areas (Connecticut, Iowa, New Mexico, Utah and Hawaii and metropolitan Detroit, Atlanta, New Orleans, Seattle and San Francisco-Oakland) with more than 10 percent of the U.S. population.

The 1,082-page report and accompanying analyses also reveal that:

The average American's risk of having cancer before age 74 is nearly one in three, and the risk of dying from it one in six.

There is little strong evidence (in the view of NCI epidemiologists) for an often-alleged "cancer epidemic," though there was a noticeable increase in breast cancers starting in 1974.

The breast-cancer case rate has started to fall, suggesting that there may simply have been a "1974 blip" caused by the increased awareness and case-finding that attended two famous breast-cancer patients: Betty Ford and Happy Rockefeller.

Cancer rates remain alarmingly high among black men (22 percent higher than among white men), although black women have slightly fewer cancers than white women do.

The fact that black women are at no greater risk of this disease than white women is still another mystery, by and large. But a new NCI analysis of the high rate of cancer of the esophagus (or gullet) in the District of Columbia shows that black men who have this kind of cancer are most commonly heavy drinkers, and, somewhat less commonly, eat a poor diet, with little meat, fish, fruit, vegetables, dairy products and eggs.

The low cancer rate of Hispanics is certain to be the subject of more study to seek common factors as well as clues to help prevent cancers in other populations.

The figures on Hispanics come from study in New Mexico and Puerto Rico, but are confirmed, said NCI's Dr. John Young, by reports from Los Angeles and South America.

Hispanic males in Puerto Rico and New Mexico alike had 38 percent fewer cancers than white males (229 compared with 371 cases per 100,000 per year). Hispanic females in New Mexico had 27 percent fewer cancers than white females; Hispanic females in Puerto Rico, 42 percent fewer.

The reason probably includes diet, said Dr. Ernst Wynder, head of the American Health Foundation. There is strong evidence that a diet high in animal and dairy fats causes an increase in some cancers, especially breast and colon cancers. Poverty and tradition have produced a Hispanic diet high in proteins from legumes (such as beans) but low in meat.

There is probably also some "under-reporting" of Hispanics' cancers. Some of their cancers may get missed entirely, because the poor, including Hispanics, often get poorer medical care.

But some types of cancer -- including cancer of the cervix, liver cancer and cancer of most of the digestive tract (that is, of the esophagus, stomach and small intestine) -- are more common in Hispanics than in whites, and are not under-reported, according to one doctor.

Dr. Isidro Martinez, Puerto Rico's director of cancer control, said Puerto Ricans' high rates of mouth, pharynx and digestive-tract cancers seem to be related to "heavy drinking of alcoholic beverages."

On the other hand, he said, "we don't have heavy industries or high air pollution" and "cigarette smoking is lower," so there is a low rate of lung cancer.

It is possible, doctors say, that as Hispanics become more like other Americans in work, income and diet, they will begin to have the same cancer patterns. This has happened with Japanese and other immigrants. But no such trend has shown up yet in Hispanics. And this may add to the evidence for some inborn, genetic protection.