The pattern of hepatitis C virus (HCV) serum markers and liver disease was investigated in 11 leukemic children showing anti-HCV reactivity at least once during long-term observation to define the role of HCV infection and the behavior of HCV serologic markers in this patient cohort. Antibodies to HCV by first- and second-generation enzyme-linked immunosorbent assay (ELISA) and by second-generation (four antigens) recombinant immunoblotting assay (RIBA) and HCV-RNA by nested polymerase chain reaction (PCR) were serially examined in serum. Liver disease was defined according to transaminase levels. Seven of 11 patients were found HCV-RNA positive during chemotherapy and after blood transfusion, 3 of 11 became viremic during follow-up, and 1 of 11 was always HCV-RNA negative. Seroconversion to anti-HCV positivity by second-generation ELISA occurred in all the HCV-RNA positive children either during or after chemotherapy. Alanine aminotransferase (ALT) levels were elevated in all the HCV-RNA positive patients during antileukemic treatment and normalized in seven of them after therapy withdrawal, despite persisting viremia. These results indicate that HCV-RNA testing by polymerase chain reaction is required to correctly identify HCV infection in patients with leukemia while on chemotherapy. Viremia did not correlate with ALT levels and anti-HCV patterns.