Sudden and unexplained death in sleep (SUDS) is a significant cause of death of young adults in several Asian populations, but its distribution and incidence are not well known. We conducted a survey by mail of SUDS (known as 'laitai' in the local dialect) that occurred in adults during 1988-1989 in 3867 villages in northeastern Thailand with a total population of 5.42 million. Headmen of 2651 villages (68.6%) returned the questionnaire and sudden deaths of adults 20-49 years old were reported in 396 of these villages. The validity of reports was assessed by interviewing next of kin and witnesses in a sample of 92 villages reporting sudden deaths; 60 of 127 reports of SUDS from these villages were verified (47.2%). Officials and villagers in seven villages that did not respond to the questionnaire were also interviewed and no cases of sudden death were found. The verified SUDS victims were all men wth a mean age of 35.9 years (SD 7.8). A family history of SUDS was reported in 40.3% of index cases and 18.3% had brothers who had died similarly; no such deaths were reported among sisters. The estimated annual rate of death from SUDS among men 20-49 years was 25.9 per 100,000 person years (95% confidence interval (CI): 21.0-30.7). The sudden deaths were seasonal with 38% occurring during March-May and 10% during September-October (chi 2 = 9.45, P = 0.02). Sudden death in sleep is a leading cause of death of young men in rural northeastern Thailand and the characteristics of Thai victims are similar to those of other Asian victims of this unexplained syndrome.