Hepatitis C virus (HCV) is considered as a causative agent for most non-A, non-B hepatitis cases. Since patients with lymphoproliferative diseases undergoing chemotherapy are highly immunosuppressed and receive multiple blood transfusions they may be at risk for HCV infection. The aim of the study was to estimate the incidence rate of HCV infection and to analyze the HCV genotypes in a three-year-follow-up study of children with leukemias and lymphomas undergoing chemotherapy. The study group consisted of 148 children, without HCV antibodies (HCVAb) at the time of diagnosis. HCVAbs were estimated with EIA test of 2nd generation (Organon Teknika). Presence of HCV-RNA particles and their genotypes were detected by nested PCR method and line probe assay (Inno-Lipa HCV II). The presence of HCVAb during therapy was documented in 42 patients (28.4%), with median time 14 months from diagnosis. Among 42 children with positive HCVAb, HCV-RNA was confirmed in 36 patients(85.7%) by PCR in every examination. 34 of HCV-RNA positive cases were infected with HCV with a single genotype, including type 1a in 31 children and type 1b in 2 patients. Coinfection with two genotypes of HCV was demonstrated in two cases (1a+1b). None of the HCV-RNA positive children has become negative during the 3-year-follow-up study. Persistence of viremia in the HCV-infected patients can be explained by the therapy-induced immunosuppression. The presence of the same HCV genotype in almost 90% of the analyzed children strongly suggests the common source of HCV infection. The infectivity of those children should be seriously considered during their course of therapy.