Two or more cases of sarcoidosis in one family is not unusual. To compare the frequencies of familial sarcoidosis in Finland and Hokkaido, Japan, and to analyse the type of associations reported, we collected data on all patients visiting hospitals for sarcoidosis in 1984 in Finland (1378 patients) and Hokkaido (208 patients), including information about familial sarcoidosis. We also analysed the familial cases seen among 571 sarcoidosis patients diagnosed at the Mjölbolsta hospital in Finland from 1955 to 1987 and among 686 Japanese patients seen in Sapporo from 1964 to 1988. In 1984, 50 sarcoidosis patients visiting Finnish hospitals and nine sarcoidosis patients in Hokkaido reported as familial cases. Of the sarcoidosis patients seen in Finland at the Mjolbolsta hospital in 1955-1987, 27 had a family member with the same disease, while this number was 20 in the Sapporo hospital in 1964-1988. Those surveys give a prevalence of familial sarcoidosis in Finland of 3.6-4.7% and in Hokkaido of 2.9-4.3%. Among familial cases, the dominating relationships were sister-brother and mother-child relationships.