Among 145 consecutive patients undergoing bone marrow transplantation (BMT) for leukemia or aplastic anemia. 30 (21%) were found positive for hepatitis B surface antigen (HBsAg) in serum either before or after BMT. Their serologic profile and clinical outcome are described. Nine out of 30 patients were HBsAg positive before BMT: four were chronic carriers and five were found HBsAg+ at transplant. Three of the former and one of the five latter patients remained persistently HBsAg+ after transplant with signs of liver disease; none developed liver failure, indicating that HBsAg positivity is not an absolute contra-indication to BMT. Among the remaining 21 patients. HBsAg was detected early (n = 12) or late (n = 9) after transplant. All 21 cleared the antigen during follow-up and liver disease was either mild and asymptomatic (nine cases) or clinically overt (12 cases), but none had life-threatening liver disease. Several HBV-infected patients were constantly seronegative for antibody to HBcAg even in the presence of active HBV replication. These results show that the serologic pattern of HBV markers in BMT patients is unpredictable. HBV infection was rarely associated with severe hepatitis and HBsAg carriage.