Reactivation of chronic infection is a serious complication during and especially after the withdrawal of cancer chemotherapy in hepatitis B virus carriers. Mortality is high, ranging from 4 to 20%. We report two cases of severe reactivation, after withdrawal of chemotherapy for chronic lymphocytic leukemia in one case and for a bladder tumor in the other. Recovery occurred with lamivudine therapy. Morbidity and mortality are common in these cases, especially when intensive chemotherapy and/or chronic hepatitis and/or precore mutants viruses are involved. Although lamivudine seems to be effective in these cases, prophylactic use has not been clearly defined and must be evaluated.