We have followed up 111 transfusion receptors in the ambulatory, for at least 180 days, in order to evaluate the occurrence of post-transfusional hepatitis and the etiological agents involved in the disease in the city of Campinas, state of São Paulo, Brazil. At the end of the study we have diagnosed this hepatitis in 18 (16.2%) subjects. Out of these 18 subjects, 16 (89%) were caused by hepatitis C virus, 1 (5.5%) caused by hepatitis B virus and 1 (5.5%) with undetermined etiology, 15 months after transfusion. The average incubation period of HCV was 71 days and 23% of the HCV positive receptors remained with increased AST/ALT for more than 6 months. Late serum conversion was observed for anti-HCV in 71.4% of the subjects, averaging 135 days after the transfusion. An ALT dosage and anti-HCV determination, 3 and 6 months after transfusion would diagnose, respectively, 71 and 93% of the cases which developed post-transfusional HCV.