In a prospective study carried out in the Netherlands (1984-86) to establish the incidence of post-transfusion hepatitis non-A, non-B (PTH-NANB) in patients undergoing open heart surgery, 393 patients received 5315 blood product transfusions. PTH-NANB developed in 9 patients (index cases); stored serum samples from these patients and from 9 control patients, matched for age, sex, and number of blood product transfusions, as well as serum samples of all implicated blood products, were selected retrospectively. Sera were tested under code with a radioimmunoassay for the detection of antibodies to hepatitis C virus (anti-HCV). PTH-NANB patients received 151 blood product transfusions and control patients 140. 4 of 9 PTH-NANB patients (3/5 chronic, 1/4 acute resolved hepatitis) and 0/9 controls seroconverted. 7 of the transfusions given to PTH-NANB patients but none of those given to control patients were anti-HCV positive. In 7 of 9 serum sets from PTH-NANB index cases plus implicated donors, either a donor or the recipient was anti-HCV positive. Among the donors implicated in transmission of PTH-NANB there was a strong correlation between raised alanine aminotransferase levels and the presence of anti-HCV antibodies.