One hundred and fifty-three patients with acute hepatitis B infection over a four-year period have been reviewed. They were predominantly young males (60%) with a mild illness. There was no indication of the source of the infection in 56 patients (36%), while 52 patients (34%) had a history of intravenous narcotic abuse. Twenty-five patients were hospital staff members, while the remainder had known environmental associations with the hepatitis B virus. Eight of the 80 patients who were followed up for longer than six months had persistent hepatitis B surface antigenaemia, and evidence of progression to chronic hepatitis.