A countrywide prospective study on open-heart surgery patients was performed between 1987 and 1989 to determine the prevalence and nature of post-transfusion hepatitis in Finland. Altogether 685 coronary by-pass operation patients, who received on average 12.3 units of blood products, were postoperatively followed for 6 months. Ten blood samples were drawn from each patient. Hepatitis was diagnosed when the alanine aminotransferase values exceeded the upper normal value 2.5 times in one sample and twice in another, and non-viral causes could reasonably be excluded. Eleven hepatitis cases (1.6%) were recorded with a mean incubation period of 8.4 weeks; all represented the non-A, non-B type. The majority had mild symptoms or were asymptomatic but two became icteric. Six patients (55%) had abnormal alanine aminotransferase values for at least 6 months, which indicates possible chronicity. These 685 open-heart surgery patients received a total of 8,436 units of blood products; thus the rate of NANBH cases per 1000 units was as low as 1.3. This is less than recently reported in six other prospective studies.