Despite preventive measures, patients who have cancer or who undergo bone marrow transplantation remain at higher risk of viral infection since they often receive multiple blood products. This category of patients also includes subjects from countries that are highly endemic for hepatitis B virus and hepatitis C virus infection and who travel to developed countries for specialized treatment. This review discusses the current opinions concerning the diagnostic, clinical, and therapeutic aspects of hepatitis B and C virus infection in different groups of patients: children with chronic infection before chemotherapy, children infected during chemotherapy or bone marrow transplantation, and patients with chronic infection after the end of treatment.