NIGHTMARES SUSPECTED IN BED DEATHS OF 18 LAOTIANS

The Federal Centers for Disease Control in Atlanta is conducting an intensive inquiry into the manner in which 18 apparently healthy Laotian refugees died mysteriously in their sleep in this country within the last four years. One possibility being explored is that they were frightened to death by nightmares.

The 17 men and a woman were members of a preliterate Laotion mountain society called the Hmong. About 35,000 Hmong are now living in the United States. Most of them fled their homeland after it was overrun in 1975 by the Pathet Lao.

The Hmong come from an isolated culture similar to that of the American Indian. Most of those who have been resettled in this country live in concentrated communities in Missoula, Mont.; Santa Ana, Calif.; Providence, R.I.; Denver and Minneapolis-St. Paul, where the largest number, 10,000 to 12,000, reside.

Very few speak English. Their own tongue became a written language only a few years ago, and their adaptation to American life has been marginal. Until some relatively recent conversions to Buddhism and Christianity, their religion is animist, governed by spirits and manifestions of the soul. Terror Induced by Nightmare

The cause of death of the 18 refugees in their own beds in the early morning hours remains a mystery. The deaths have generally been attributed to ''probable cardiac arrythmia,'' or irregular heartbeat. Although pathologists have been reluctant to advance it publicly, one possibility being explored is an obscure pattern described in medical literature as ''Oriental nightmare death syndrome,'' in which death results from terror induced by a nightmare.

Dr. Roy Baron, an epidemiologist at the Centers for Disease Control, the Federal agency responsible for analyzing patterns of mysterious death that might point to a potential source of epidemic, acknowledged that nightmares were a possibility but emphasized that no conclusion had been reached.

However, Dr. Baron did say that, on the basis of reports from medical examiners who had examined the bodies, preliminary analysis pointed to ''natural causes'' and, in effect, ruled out accidents, foul play and suicide.

''There is all kind of speculation, but for the time being, we have no real explanation,'' he said. ''All these people seem to have been in good health.''

Dr. Michael McGee, assistant medical examiner for Ramsey County, Minn., which includes St. Paul, where four of the deaths occurred - a fifth occurred in the sister city of Minneapolis - was more explict. Doctors Begin to Wonder

''I know what they didn't die of,'' he said. ''They didn't die of getting shot in the head, stabbed in the heart; they didn't fall off the roof; they didn't get poisoned; because we did an autopsy in each case, and we got a big zero.''

He noted that the five deaths in the Minneapolis-St. Paul area had occurred within 18 months. ''We didn't think anything mysterious was afoot until the third and fourth deaths happened very quickly,'' Dr. McGee said, ''but then we began to wonder.

He said that his own search of the medical literature had disclosed the startling possibility that the four who died in St. Paul, all apparently healthy young men, might literally have been frightened to death.

The syndrome is called ''bangungut,'' a Filipino word for ''nightmare,'' and is described in medical literature as ''nightmare death syndrome.'' The fatal affliction, Dr. McGee said, has been suggested as the cause of death in similar cases among young Filipino males between 30 and 40 years old.