A retrospective study of hepatitis C virus (HCV) transmission by transfusion was conducted in Orebro county. Out of the 7,900 active, registered blood donors, 21 repeatedly anti-HCV reactive (RIVA 2 positive or indeterminate) donors were diagnosed. Their 84 recipients from January 1990 through June 1992 were identified and 41 (49%) were alive in December 1992. A total of 13 anti-HCV reactive (RIBA 2 positive or indeterminate) were diagnosed in 39 investigated recipients. Of these 11 were previously undiagnosed, and seven were HCV RNA-positive. In the donor population 1.03% were anti-HCV-positive by ELISA, but only 0.09% were RIBA and HCV RNA-positive. In 1990, 0.06% of the blood components came from the HCV RNA-positive donors, and none during the first 6 months of 1992. In order to identify transfusion-transmitted HCV infections that took place before the introduction of tests for anti-HCV antibodies, patients with a history of transfusion and symptoms and signs of liver dysfunction or damage should be thoroughly tested.