Seventy-four patients in 1978 and 316 in 1986, all transfused during open-heart surgery in Stockholm, Sweden, were studied prospectively for the development of posttransfusion non-A, non-B (NANB) hepatitis, seroconversion to hepatitis C virus antibodies (anti-HCV) (C-100), time lag to seroconversion to anti-HCV and outcome of posttransfusion NANB/C hepatitis. Anti-HCV was tested up to six months after transfusions in patients from 1978 and up to one year after transfusions in patients from 1986. Fifty-four percent of the patients who developed posttransfusion NANB hepatitis seroconverted to anti-HCV, 7/15 (47%) in 1978 and 8/13 (62%) in 1986. Four (27%) of the 15 patients who seroconverted to anti-HCV were anti-HCV reactive within one week, 12 (80%) within eight weeks and all within 18 weeks after the onset of hepatitis. The ELISA optical density/cut-off (OD/CO) ratio was above 4.0 in all patients with hepatitis C who seroconverted. One transfused patient with normal serum aminotransferase levels throughout follow-up seroconverted after six months. He had a temporary positive anti-HCV reactivity with a maximal ELISA OD/CO ratio for anti-HCV of only 1.2, which became negative three years later. Development of chronic hepatitis was noticed in 9/15 (60%) patients who seroconverted to anti-HCV and in 5/13 (38%) patients with posttransfusion NANB hepatitis who did not seroconvert.