Treatment of patients with neoplastic diseases of the lymphatic or lymphoreticular system and HBV infection can lead to reactivation of viral infection. Assessment of HBs antigen among this group is insufficient for the diagnosis of chronic HBV infection. Current research suggests the necessity of determining anti-HBc, antiHBs and HBV-DNA. Elimination of HBV as well as the influence of the virus on hepatocytes is associated with increased inflammatory and necrotic changes in the liver. Understandable, therefore, becomes a possibility of significant damage to hepatocytes caused by HBV during chemoimmunotherapy. Ofparticular importance in the reactivation of HBV are glicocorticosteroids acting as suppressants of the immune system and rituximab activating B cell apoptosis. Reactivation of HBV may occur in more than 60% of patients with positive HBs antigen and in approximately 50% of patients without HBsAg. Early therapy with nucleo(z)tide analogues significantly reduces the incidence ofHBV reactivation.