Over a four-year period in a five-county area, 71 patients with clinical hepatitis B had dental work performed in the two to six months before their illness. Fifty-five cases were traced to a single oral surgeon. Seventy-nine per cent of these patients were positive for hepatitis B surface antigen (HBsAg) and most had no other recognized source of hepatitis. An investigation of the implicated dentist uncovered no gross inadequacies in instrument sterilization or general dental procedures; however, the dentist was found to be an asymptomatic carrier of HBsAg of the same subtype (ay) as nine of 11 of his patients who had hepatitis and whose serums were available for testing. Since no HBSAg was found in the dentist's saliva, urine or feces, and since he admitted to having frequent cuts on his fingers, it is possible that a "hemoral" transmission of HBs Ag produced hepatitis in his patients.