Sudden unexpected death is generally considered to be caused by acute myocardial infarction and/or arrhythmia. To document the incidence and causes of sudden death in Japan, where the incidence of myocardial infarction is low, the present study examined death certificates, hospital records, the forensic medical records, and the police records of residents of the southern part of Okinawa island who died at the age of 20-74 years during a 3-year period from January 1, 1992 to December 31, 1994. Sudden death was defined as death within 24 h from the onset of unexpected symptoms. The study documented 126 (87 men and 39 women) sudden deaths. The crude incidence rate was 0.37/1,000 person per year (0.51 in men and 0.23 in women). According to the death certificates, 78 cases died of heart diseases. However, the cause of death could be determined by examination of all available records in only 64 cases: myocardial infarction in 10, non-ischemic heart diseases in 13, and stroke in 23 cases. Even when the analysis was limited to the cases who died within 1 h from the onset of symptoms, heart disease was the cause of death in only 22% of the cases while the cause of death could not be determined in 53% of the cases. Only 13% of those diagnosed as heart diseases on the death certificate were verified. The agreement rate between the diagnosis reached by the re-evaluation of the records and that on the death certificate was 82% for stroke and 33% for other diseases. In Okinawa, Japan, the frequencies of heart disease and stroke as the cause of sudden death may be similar. Except for stroke, the diagnosis appearing on the death certificate has substantial inaccuracy. (Jpn Circ J 1999; 63: 37 - 42)