BACKGROUND: The objective was to determine the contribution of transfusion in the past to the risk of current infection with hepatitis B or C among patients attending a large hospital for endoscopic procedures. STUDY DESIGN AND METHODS: Blood samples had been tested for hepatitis markers by routine methods. Patients completed a comprehensive risk factor questionnaire and results were analyzed using computer software. RESULTS: Twenty-seven percent of the 2120 participants in the study received transfusions in the past. There was no increase in prevalence of hepatitis B among those transfused. Compared with nontransfused participants, recipients of blood before the implementation of hepatitis C virus (HCV) screening in 1990 had a 4.6-fold increased risk of HCV infection, whereas those transfused with screened blood had a 3-fold increased risk. The difference between the odds ratios for patients before and after screening was not significant. CONCLUSIONS: Because screening has almost completely eliminated HCV from the blood supply, our finding of a continuing association of HCV infection with transfusion was unexpected. It implies that there are significant other nosocomial risks for hepatitis C transmission associated with the clinical situations where patients received blood. These should be actively investigated.