A nested polymerase chain reaction was used to assess viraemia in blood transfusion recipients with no serological evidence of hepatitis C virus (HCV) infection (naive recipients) and in recipients with prior or existing HCV infection (infected recipients), who were transfused with HCV-positive blood. In 10 hepatitis cases in naive recipients, defined as primary infection, nine showed clinical hepatitis, and one was sub-clinical; the time between transfusion and elevation of alanine aminotransferase (ALT) levels was 15-60 days (37.9 +/- 13.9). All 10 naive recipients showed abnormal ALT, and 10/10 and 7/10 were persistently positive for anti-HCV and HCV-RNA, respectively, for more than 1 year. Similarly, in five cases in previously infected recipients, defined as re-infection, 4/5 showed clinical hepatitis, the time to elevation of ALT was 30-46 days (34.8 +/- 6.4), and 5/5 and 3/5 were persistently positive for anti-HCV and HCV-RNA, respectively, for more than 1 year. All five infected recipients showed abnormal ALT. In conclusion, there was no significant difference (P = 0.05) in the frequency of the markers of infection resulting from primary or re-infection with HCV, suggesting that primary infection fails to induce a protective immune response.