Eight antibody-positive individuals were detected among 12,000 blood donations during the first year of screening blood donors for hepatitis C virus (HCV) antibodies in Iceland. All 8 were found to have a history of intravenous drug abuse. Six of these 8 individuals had previously donated blood to 27 patients who could be traced and examined for HCV infection. The great majority (23/27, 85%) of the recipients had demonstrable HCV antibodies. Furthermore, RNA analysis with the polymerase chain reaction showed that all patients with HCV antibodies had HCV RNA in their serum and in one hemodialysis patient without HCV antibodies viral RNA could be demonstrated. Genotyping of the HCV strains showed that the genotype of the donor was also identified in all but one of the infected recipients of his/her blood or blood products. This study, therefore, substantiates high infectivity of the HCV by blood or blood factor donation and shows that viremic HCV antibody-negative individuals exist.