In the period July 15, 1977, through March 30, 1982, there were at least 51 sudden, unexplained deaths in the United States among refugees from Southeast Asia. These deaths involved relatively young (median age, 33 years), previously healthy persons. All except one were male, and all died at night. Available data from studies among young adults in this country suggest that this specific phenomenon has an unusually high incidence among Laotian and Kampuchean refugees and, furthermore, has not previously been observed in the United States. Interviews with families of the decedents and a case-control study have failed to establish causal factors, but emotional stress cannot be ruled out as a contributing element. While reviews of the forensic investigations have confirmed the absence of important common pathological or toxicological findings, preliminary findings of special postmortem cardiac studies, now in progress, suggest that at least some of these deaths may be associated with developmentally abnormal conduction system pathways. Further studies are required to confirm this association, to define the apparently sleep-induced mechanism that triggers these deaths, and to explain the male preponderance in this disorder.