Dozens of southwest Utah residents have died of cancer -- and many more deaths may follow -- because of unmonitored fallout from 31 years of nuclear bomb tests in the Nevada desert, a medical expert testified in federal court here today.

The testimony from Dr. Carl J. Johnson highlighted a multimillion-dollar suit against the federal government brought on behalf of relatives of 24 alleged fallout victims from tiny communities in southwest Utah. The government, scheduled to begin a lengthy defense next week, contends that the cancers could have had other causes.

Johnson, a Denver physician and epidemiologist, said his study of more than 4,000 persons living in Utah towns such as St. George, Parowan and Paragonah showed 288 cases of cancer during two nine-year periods when according to previous cancer studies only 179 would have been expected. He said it was particularly significant that the most prevalent cancers were associated with radiation, such as leukemia, which occurred four times more often than expected, and thyroid cancer, which occurred six times more often than expected.

A report by Johnson said studies of victims of the 1945 atomic bombings of Hiroshima and Nagasaki in Japan showed very long dormant periods before radiation-caused cancers broke out. "It seems probable that the largest increment of cancer in southwest Utah is yet to come," the report said.

U.S. Energy Department attorney Jake Chavez challenged Johnson's testimony before it began yesterday and cross-examined him closely today. Chavez said the study was based on "unreliable data" because Johnson did not get medical records to back up testimony from relatives of cancer victims.

Testimony in the four-week trial indicates that officials delayed the nuclear bomb tests when winds would have taken the fallout toward heavily populated Las Vegas and southern California, but that they appeared to have paid little heed to the effects of fallout on little Utah towns about 120 miles east of the blast site.

Fallout-monitoring devices were scattered and sometimes not turned on at the proper time, according to witnesses. One former fallout-monitoring official testified that a warning of a fallout cloud detected after a 1953 blast came one hour after the cloud had already passed over St. George, which then had a population of 5,000 and now has 11,350 people.

Claims from 1,178 relatives and surviving victims allegedly affected by fallout have been filed against the government, according to St. George attorney MacArthur Wright. The current trial, covering 24 affected individuals, only five of them still alive, is exploring uncharted legal ground. Its conclusion may determine how attorneys handle the remaining cases.

U.S. District Court Judge Bruce S. Jenkins said early in the trial he was certain the case would be appealed to the Supreme Court no matter who won. But he agreed to go ahead, despite a government request that the case be dismissed on grounds that testing officials were acting with "legal discretion" and thus not liable for suit under the law.

Henry Gill, an Energy Department attorney leading the defense, said in his opening statement that the government witnesses would show that, "to a high degree of medical certainty, the claimed injuries are not caused" by fallout. He said there was great uncertainty about the effects of low-level radiation.

Of the 100 people packed in the courtroom on that opening day, "20 of us will die from cancer," he said, but cancer caused by radiation would be identical to cancers from other causes.

Lisa Pectol, one of the victims mentioned today, was 20 years old and five months pregnant with her first child in 1976 when doctors discovered she had a brain tumor. Three days later she was dead. She had grown up in the area allegedly affected by fallout, and Dr. Johnson said today he believed her cancer was "caused by her exposure to radiation."

Her husband, Dwight Pectol, said in a telephone interview from his home in Washington, Utah, that he joined with other area residents four years ago in organizing a suit "to bring it out and let people know what was going on."

Now 29 and the owner of a store selling pre-hung doors, Pectol said he remains upset that underground nuclear tests, which occasionally leak radiation to the surface, still continue in Nevada. "They say on the radio it's going to blow, but they never say if anything leaks," he said. He now has a 2-year-old son by a second marriage and said, "I wonder what's going to happen to him."

Although there had been speculation and some expressed worry in the area about fallout from the beginning of the tests in 1951, federal officials regularly discounted the danger. One Nevada politician called fallout complaints in 1957 "communist-inspired scare stories." Helen Nisson, whose 13-year-old son Sheldon died in 1959 of leukemia, testified earlier in the trial that "we believed the government. The government was the most marvelous thing. We thought [it] wouldn't do anything wrong."

Experts have testified that much of the radiation from the fallout was absorbed into the milk of local cows, greatly increasing the danger to children who drank the milk. Sheep in the area also suffered from radiation burns and sickness. A federal judge earlier this year accused some government officials of "a fraud on the courts" for suppressing evidence and pressuring witnesses to cover up a connection between fallout and the 1953 death of about 4,000 sheep.