We conducted a long-term prospective study of 89 cancer survivor children who had acquired hepatitis B virus (HBV) and/or hepatitis C virus (HCV) during treatment for neoplasia, the aim being to evaluate the natural history of the diseases and the effect of interferon (IFN) treatment. Patients were followed up for a median period of 13 years (range, 8 to 20); 46 were infected by HBV, 11 by HCV, and 32 coinfected by HBV and HCV. A spontaneous clearance of hepatitis B surface antigen (HBsAg) occurred more frequently in coinfected patients (19%) than in the HBV-infected (2%; P =.004), with an annual seroconversion rate of 2.1% and 0.2%, respectively (P =.008). Loss of hepatitis Be antigen (HBeAg) occurred in 44% of coinfected and in 28% of HBV-infected patients. Clearance of serum HCV-RNA was observed in 34% and 9%, respectively, of coinfected and HCV-infected patients. Seventeen HBV-infected, 4 HCV-infected, and 16 coinfected patients received alpha-IFN treatment. In the HBV group, 6 patients (35%) cleared serum HBV DNA and seroconverted to anti-HBe; in the HCV-group, none cleared HCV-RNA. In the coinfected group, 1 patient cleared both HBV DNA and HCV-RNA, 6 patients cleared serum HCV-RNA alone, and 1 only HBV DNA and HBeAg. Overall, the diseases showed a mild histological course with no evidence of liver cirrhosis. A reciprocal interference on viral replication between HBV and HCV may occur in coinfected patients. Treatment seems to be effective for selected cases and is justified in view of the uncertain prognosis of the disease in these patients.