Children who grew up in southern Utah in the 1950s, when the area was dusted repeatedly with radioactive fallout from atomic bomb tests, died from leukemia at more than twice the normal rate, a major new medical study has found.

That part of the state received the heaviest doses of radiation, but the study also uncovered evidence that children farther downwind in northern Utah also experienced higher rates of leukemia than normal.

The study, conducted over the past year by a group of University of Utah health investigators here, compared leukemia rates among children who lived in the area during the atomic tests and those living there before and after.

The study is to be published Thursday in the New England Journal of Medicine.

It is the first effort not controlled by the federal government to assess scientifically the health effects of more than 90 nuclear blasts at the Nevada proving grounds, and it provides the strongest link yet between the A-bomb tests and illnesses downwind.

Previous studies by the federal government either pointed toward similar findings and were suppressed, or were restricted and their funding cut off before they could be concluded on a strong scientific basis.

The findings, principally the work of Dr. Joseph L. Lyon, an epidemiologist who runs the state's cancer registry, come at a time of growing concern over the effects of low-level radiation -- and mounting claims totalling tens of millions of dollars by falloutzone residents who say they lost loved ones to the testing program.

Dr. Lyon said today that the study found a strong association between fallout and a sharp, temporary increase in childhood leukemia that occured statewide but primarily in heavy fallout areas. He said he expects continuing efforts to study the problem, possibly a major U.S. government-funded undertaking, to make that tie even stronger, given the limitations on this investigation.

Dr. Lyon had previously told The Washington Post that the fallout-lekemia link could mean that residents in the fallout zone received more radiation than the old Atomic Energy Commission claimed.

Or, he said, it may show that it takes less radiation ta cause leukemia than previously believed.

While Lyon's study was confined to Utah, it raises major questions about what happened to thousands of other Nevada and Arizona residents living in what AEC said were "virtually uninhabited" areas outside the nuclear test site northwest of Las Vegas.

At least one Arizona community has reported a rash of leukemia after the testing began, and some residents of Nevada claimed years ago they suffered leukemia and othre cancers as a result of their exposure to fallout.

The Lyon group studied Utah death certificates from 1944 to 1975, sorting out cancers among children under 15. These were divided into two groups -- leukemia, which can be associated with radiation, and all other cancers. They found that before the late 1950s, 14 high-fallout Utah counties consistently had a lower rate of childhood leukemia deaths than the rest of the state or the United States as a whole.

The same was true for the 1967-to-1975 period. But in between, "a signifired in children up to 14 years of age living in Utah," the study found. "This excess was concentrated [among] children born between 1951 and 1958, and was most pronounced in those residing in counties receiving high fallout."

Moreover, the Lyon group concluded, those children who had lived in the high-fallout areas the longest -- those born about 1951 -- experienced "the treatest excess" of actual leukemia deaths compared to statistical expectation.

The study found a somewhat excessive number of leukemia deaths among those born around 1965 and "little excess" among those born around 1961.

All told, the state experienced 184 leukemia deaths among children during the study period -- 52 more than normal. Nineteen of those 52 "excessive" deaths occurred in the sparsely populated high-fallout counties near St. George, Utah; there 32 children died when only 13 would have been expected to.

The other 33 "excessive deaths" were scattered across the state, meaning that those areas also may have had children affected by fallout or that some children from the heavy fallout areas perhaps moved there.

Lyon notes that the heavy-fallout counties have been losing population areas of Utah, despite a high birth rate. Thus his findings may actually understate the number of leukemia deaths that could be associated with fallout.

While leaving open the possibility that other factors may have caused the temporary rise in childhood leukemia deaths in Utah, Lyon said in an interview it would be "highly unusual" for some environmental factors to hit just one age group -- the children born in the 1950s -- and then disappear.

The Lyon group estimated that, given the number of excessive deaths, it may not have taken much fallout to cause leukemia -- perhaps only 50 percent more radiation than the AEC has admitted. AEC estimates of radiation exposure have been challenged by many as too low.

Lyon, like other researchers, said there is a lack of data to tell accurately how much radiation people in the fallout zone were exposed to.

The study found a decrease in other childhood cancers.

An editorial by Charles E. Land of the National Cancer Institute, printed in the New England Journal observed "it is unlikely" that fallout caused an increase in leukemia deaths at the same time as a decrease in other cancers was occurring.

Land also said that the sharp, temporary increase in leukemia deaths appears high because of the low occurrence of leukemia in the counties that received little or no fallout.

He urged further study, which Lyon said probably would confirm the findings.