Sudden death during sleep has occurred among previously healthy Southeast Asian male refugees, but routine autopsies have not determined the cause of death in any of these cases. We report the first systematic attempt to define the cardiac abnormalities associated with this syndrome. Among 18 hearts examined, 14 showed slight to significant cardiomegaly, characteristic of increased cardiac work load. The reasons for the cardiomegaly remain unexplained. Conduction system anomalies were present in all but one heart. These included persistent fetal dispersion of the atrioventricular node and/or bundle of His, present in 14 hearts; accessory conduction fiber connections, found in 13 cases; and congenital heart block, observed in one case. These abnormalities were associated with variations in the structure of the cardiac base, suggesting a common aberrant developmental process. Although the functional significance of these findings has not been established, the conduction system anomalies may be the substrate for sleep-related cardiac arrhythmias and sudden death.