From 1992 to 2001 hepatitis C virus (HCV) viremia was studied in 53 renal transplant recipients anti-HCV+ with at least 3 months follow-up posttransplant using a quantitative retrotranscriptase-PCR method. HCV-RNA was detected in 45 (85%): 29 of the 34 recipients treated with azathioprine-based therapy and 15 of 18 treated with mycophenolate mofetil. Immunosuppressive therapy type did not affect HCV replication. Three different patterns of HCV-RNA evolution were detected: 13 (28.8%) patients with high RNA-HCV levels; 21 (46.7%) patients with low levels; and 11 (24.4%) patients with viremia elevation. In 10 (90%) of 11 of the last group, HCV viremia was detected before 15 days posttransplantation, significantly earlier than in the other two groups. Thus, replication during the first 15 days after transplantation leads to a high RNA-HCV viral load. No clinical symptoms were related to HCV.