Enzyme immunoassay has been used in 102 patients with acute leukemia (AL) and 340 donors to identify serological markers of hepatitis B virus (HBV) and hepatitis C virus (HCV): HB-sAg, anti-HBs, anti-HBc, anti-HBc IgM, HBeAg, anti-HBe. At the moment of AL diagnosis in 50 patients with acute myeloid leukemia and in 28 patients with acute lymphoblastic leukemia at least one HBV marker was found in 68% and 71%, respectively, HBsAg in 12% of patients with AML and 25% of those with ALL. In remission maintenance, HBsAg was registered in 83% of patients with AML and 44% of those with ALL. Anti-HCV in diagnosis of acute leukemia were detected in 1.28% of patients, 12 months after in 40% with ALL and 67% with AML. In examination of the donors at least one HBV marker was found in 36%, HBsAg in 2.1%, anti-HCV in 1.86% of the examinees. It is inferred that acute leukemia patients are often infected with HBV and in long-term follow-up demonstrate reactivation of viral infection, though infection with mutant B viruses is also possible. HCV is a main cause of posttransfusion hepatitis in this group of patients.