To investigate the proportion by type of stroke in communities, a stroke surveillance was conducted in three rural populations between 1979 and 1987. Among 411 stroke patients aged 40 and over determined by the modified Millikan's stroke criteria, the type of stroke was analyzed using computed tomography (CT) criteria for 273 patients who were examined by CT. 1. Incidence rates of stroke determined by the modified Millikan's criteria did not vary among the three populations in all age-sex groups except for men aged 60-69. 2. The proportion of stroke patients who had CT examinations was 84-88% for men and women aged 40-69 in the three communities. 3. According to the CT criteria, the proportion by type of stroke in patients aged 40-69 in the three communities was 32% for cerebral hemorrhage, 16% for subarachnoid hemorrhage, 42% for cerebral infarction and 10% for unspecified stroke without CT abnormalities. Among the total with cerebral infarction, the proportion with cerebral infarction in penetrating artery regions, mostly lacunar infarction, was 65% and that of infarction in cortical artery regions, mostly thromboembolic infarction, was 35% for patients aged 40-69. 4. Cerebral infarction in cortical artery regions was classified further into embolic, thrombotic and unspecified types based on CT findings, the presence of embolic origin, and symptoms at the onset. The proportion of these three types of infarction were similar in patients aged 40-69. Compared with hospital-based studies in Japan, the present population-based study showed a higher proportion of cerebral infarction and infarction in penetrating artery regions. This study also indicated that the proportion of cerebral hemorrhage and infarction in penetrating artery regions was higher and that of infarction in cortical artery regions lower in Japanese than in Caucasians.