In search of freedom, opportunity and a new life, Yong Leng Thao brought his wife and eight young children to America from a Laotian refugee camp in Thailand last July. His search ended last month.

Yong Leng Thao touched off -- with his death -- a medical mystery, one that has left this city's and the nation's Hmong tribal communities in fear and prompted a call for a federal investigation.

He had been up late watching television with an uncle and slipped into bed in the early hours of Jan. 8. That briefly awoke his wife, Xiong You, but both were soon asleep.

Then came his labored breathing, so loud that it awakened her. She shook him. the tears ran down her face as she recounted those next moments of horror, moments in which she realized that she could do nothing more.

Suddenly, her healthy, 47-year-old husband was dead.

Yong Leng Thao's death would have gone as unremarked as several other Hmongs' already had, except that he was the second Hmong in Portland to pass across Dr. Larry V. Lewman's autopsy slab in three days.

And for both Yong Leng Thao and Xiong Tou Xiong, 29, there was no explanation, no suggestion of the cause of death, not even after an autopsy, microsopic examination of tissue, toxicology tests and lengthy interviews with surviving relatives.

"Pending," Lewman, the county medical examiner, wrote of the cause of the deaths.

Then he learned that two other morgue pathologists here had seen two other such deaths of Laotian refugees last year. Thus, in nine months, four men were dead with no medical explanation.

"All were very, very similar," says Lewman. "They were restricted to males, all relatively young, who died during their sleep.

"I don't know what we've got."

He called medical officials in other cities that have large Hmong and Indochinese populations, seeking indications of a more widespread pattern of death. He found none -- until Minneapolis officials told him that although they had not had any mysterious deaths of Hmongs, St. Paul had; four among the area's Hmong population of 8,000 had died mysteriously.

That spurred the search for more. Last week, after another Hmong man's death in Minneapolis on Feb. 17, Lewman had recorded at least 13 sudden, unexplained deaths since 1978 of sleeping Laotian Refugees.

Within the Hmong communities, which account for most of the Laotian refugee mystery deaths, there is talk of still other deaths. "There have been 19, 20, very similar deaths," says Kuxeng Yongchu, a Hmong leader here.

Some attribute the deaths to chemcial agents that the Hmong and other hill tribes were exposed to during the prolonged warfare of Southeast Asia. Vang Pao, a Hmong tribal leader who lives near Missoula, Mont., has been quoted as saying the deaths are "almost certainly" linked to chemical nerve agents used against the Hmong, many of whom once worked for the CIA.

"It just doesn't make sense to me," said Lewman, disputing the claim. "Nerve gas doesn't act this way. There's no evidence. . . .

"Secondly, if it was nerve gas, why does it affect only males, and why only during the night? And if nerve gas doesn't affect people right away, if it takes four years, it's not very effective."

Nonetheless, Rep. Jim Leach (R-Iowa) has asked for a federal investigation of the deaths.

Lewman wonders whether the remote mountain people have long experienced such swift, inexplicable death, which only now, for the first time, is being scrutinized by modern medicine.

"We do see sudden, unexplained death in younger people every year," Lewman said, "maybe four, five, six in a population of a million. But four out of 2,000 (refugees Portland) is way out of whack. We don't know whether this occurs in the native country to this extent."

He cites "bangungut syndrome" -- named after the Filipino word for nightmare -- that is known to strike and kill young Filipino men in what appears to be a similar manner. But for an explanation of the deaths themselves -- if not their bizarre frequency -- he observes:

"The way in which they died, suddenly, suggests it was the heart, a cardiac [nerve] conduction problem."

"All in their sleep suggests a defect in control of respiration, respiratory arrest."

But neither leaves any physical evidence.

"In our village," says widow Xiong You, "we have never had such a drama" like the deaths of her husband and the others.