In a series of 6 multitransfused, immunocompromised patients, the diagnosis of posttransfusion hepatitis C was based upon the analysis of long-term follow-up serum samples. The HCV RNA was detected by a nested PCR assay using primers located in the 5' noncoding region (5'NCR), and anti-HCV antibodies were assayed with third-generation tests. The interval between the first rise in alanine aminotransferase and seroconversion varied from less than 5 months to more than 38 months. Five out of 6 patients seroconverted after 14 months or later. In most cases, the anti-NS3 and anti-NS4 antibodies appeared first. In such patients, the etiology of chronic liver disease may thus be overlooked for 1 or more years, a definitive diagnosis requiring the detection of HCV RNA.