We studied the frequency and time of appearance of antibodies to the hepatitis C virus (HCV) retrospectively in the sera of 127 patients who underwent heart surgery between 1983 and 1986. They received blood from volunteer donors hepatitis B surface antigen (HBsAg) negative with normal serum alanine-aminotransferase levels. A prospective follow-up was carried out every 15 days for at least 6 months from the moment of the transfusion. Of the ten patients who developed biochemical criteria of post-transfusional non-A non-B hepatitis, six seroconverted to anti-HCV (60%). Of the other 117, two were already positive before transfusion (1.51%), one patient showed antibodies only in the first post-transfusional serum (passive transfer), and another two patients with no evidence of post-transfusional hepatitis developed HCV antibodies on the 90th day, remaining indefinitely (afterwards seroconversion without hepatitis); both patients' earlier sera were anti-HCV negative. Four (40%) of the ten patients with post-transfusional hepatitis did not develop any serum markers to known hepatotropic agents. Although these findings do not exclude a viral infection by these viruses, they are consistent with the involvement of an unidentified non-A, non-B, non-C agent.