Between March 1986 and September 1990, 67 of 243 cardiac transplant recipients in outpatient care at our clinic became hepatitis B virus surface antigen (HBsAg) positive after operation. The HBsAg of 63 patients belonged to the subtype ay, suggesting a common source of infection. These 63 cases and 103 controls with negative hepatitis B virus (HBV) serology were studied in order to analyse the outbreak. The sources of infection were patients who were chronic HBsAg carriers. Infection was transmitted at the time of endomyocardial biopsy, if performed on the same day and in the same room after biopsy of an HBsAg positive patient. The most likely mode of HBV transmission was droplet contamination of instruments and/or medication vials used for subsequent patients. Performing biopsies on HBsAg positive and negative patients in separate rooms resulted in the termination of the outbreak.