In Nigeria, where the carrier rate of hepatitis B antigen (HBsAg) is between 5 and 10%, it must be assumed that the instrument used for upper gastrointestinal endoscopy has had contact with an HBsAg-positive patient. The risk of type B hepatitis transmission via upper gastrointestinal endoscopy was evaluated, therefore, in 61 patients who were followed for 1 year after the procedure. Four of these patients were "carriers" of HBsAg. None of the study group developed jaundice or hepatomegaly during the follow-up period. In 21 patients, serial liver function tests and screening for HBsAg were obtained. None of the 19 seronegative patients developed abnormal liver function tests or converted to HBsAg-positive. It is concluded that, while transmission of HBsAg is theoretically possible, the risk appears minimal. HBsAg positivity should not be a contraindication, therefore, in patients who need endoscopic assessment for definitive management.