In this study, we tested 137 Turkish children with cancer (51 with acute leukemia, 48 with lymphoma, 38 with solid tumors) while they were undergoing chemotherapy, and a control group of 45 for evidence of hepatitis B virus (HBV) and hepatitis C virus (HCV) infections using the enzyme-linked immunosorbent assay (ELISA) and polymerase chain reaction (PCR). The control group included children with other disease who had applied to the outpatient clinic during the study period and had no history of jaundice or transfusion. Sixty-five (47.4%) patients with cancer and 7 (20%) children in the control group were positive for hepatitis B surface antigen (HBsAg) (p < 0.01). HBV DNA was detected in 59 (43.1%) patients and in 9 (20%) controls (p < 0.01). HCV specific antibody (anti-HCV) was detected in 8 (5.8%) patients and in 1 (2.2%) control (p > 0.05). Eight patients (5.8%) had circulating HCV RNA, but none had in the control group (p = 0.09). Ten (13.9%) of the 72 patients who were negative for HBsAg had circulating HBV DNA, and 7 (5.4%) of the 129 patients who were negative for anti-HCV had circulating HCV RNA. We concluded that HBV and HCV infections are common among Turkish children with cancer. In countries where HBV infection is widespread among the general population as in Turkey, children with cancer are under greater risk for HBV infection.