Six hundred and fifty-one patients with acute viral hepatitis were identified serologically between January 1976 and December 1985. Of these, 109 (17%) had hepatitis A, 135 (21%) had hepatitis B, and 407 (62%) had hepatitis non-A, non-B. The possible infectious causes for acquisition of viral hepatitis occurring within 6 months before the onset of hepatitis were analysed. Approximately 80% of cases of hepatitis A and 70% of hepatitis B had no known risk factor, while in 67% of cases of hepatitis non-A, non-B possible risk factors for infection were documented. Infectious causes for hepatitis A were ingestion of raw shellfish (11%) and previous familial contact with patients with hepatitis A (10%). For hepatitis B, risk factors included medicare (24%), such as transfusion, surgical operation, accidental needle stick and acupuncture, and sexual contact (6%). For hepatitis non-A, non-B, the most important infectious cause was medical procedures (65%). The numbers of hospital employees were 2 (2%) with hepatitis A, 15 (11%) with hepatitis B and 14 (3%) with hepatitis non-A, non-B. These data suggest that hepatitis non-A, non-B can be a kind of nosocomial disease.