Possible routes for transmission of acute hepatitis B were studied retrospectively in 78 consecutive adult patients seen at the Department of Infectious Diseases, Roslagstull Hospital in Stockholm. Sexual transmission was found to be a major route of transmission, being more common than intravenous drug abuse. A single possible route of transmission was found in 66/78 (84%) patients. Eleven of the 78 patients (14%) had two possible routes, sexual contact being one. Overall sexual contact possibly accounted for 53% of all cases of hepatitis B, homosexual contact being responsible for only 10%. Cases reported earlier as being of unknown origin or associated with a recent visit abroad or to be 'social contact cases' are probably most often due to heterosexual transmission. Seven patients (9%) were heterosexually infected by persons who had been recently receiving medical care for hepatitis B. Seven sexually transmitted cases of acute clinical hepatitis B secondary to the patients studied were seen also. These findings show that sexual transmission, mainly heterosexual, is a major route for transmission of hepatitis B in a western society. They also emphasise the importance of taking an adequate sexual history as a prerequisite for providing effective prophylaxis for sexual partners of patients with acute hepatitis B.