Introduction. Reactivation of chronic hepatitis B virus (HBV) infection often occurs in hepatitis B surface antigen (HBsAg) positive patients undergoing immunosuppressive or chemotherapy, but can also occur in HBsAg negative, anti-HB core positive patients. Treatment of HBV reactivation with lamivudin results in favourable outcome in the majority of patients. The aim of the authors was to show the effect of lamivudin therapy to HBV reactivation caused by immunosuppressive therapy. Outline of Cases. The first patient was a 35-year-old woman with chronic hepatitis B virus infection who underwent prednisolone therapy for pulmonal sarcoidosis. Four months after the beginning of the therapy she presented with jaundice and a significant increase in serum aminotransferase level. Liver biopsy showed chronic viral B hepatitis of strong activity in the stage of rapidly developed cirrhosis. The patient was treated with lamivudine with slow reduction of prednisolone doses, which resulted in full clinical and biochemical recovery. The second patient was a 40-year-old HBsAg negative female with a previous history of resolved acute B hepatitis who received chemotherapy for non-Hodgkin lymphoma. After the third cycle of chemotherapy a significant increase in aminotransferase level occurred, chemotherapy was discontinued, but aminotransferase level still increased. At that moment she was found to be HBsAg positive, and PCR analysis detected a high viral load. Lamivudine treatment resulted in the patient’s recovery and allowed further chemotherapy. Conclusion. In case of the reactivation of chronic HBV infection during immunosuppressive therapy, it should be stopped and antiviral therapy should be immediately initiated. The use of lamivudine results in rapid suppression of serum HBV DNA, improves the outcome and enables the continuation of immunosuppressive and chemotherapy.