Over the past four decades, chemotherapy has played an important role in prolonging survival in breast cancer patients. However, it may also result in undesirable side effects such as hepatitis B virus (HBV) reactivation seen in this study. With the increasing use of chemotherapy paralleling the rise in breast cancer incidence, the occurrence of HBV reactivation is likely to further increase. Several strategies use lamivudine to deal with this problem. Initially, lamivudine had been used to treat patients who developed alanine transaminase elevation attributable to HBV reactivation during chemotherapy. However, using this strategy, fatal reactivation has also been reported. Later studies have suggested that prophylactic lamivudine significantly reduces HBV reactivation and its associated morbidity. However, these studies were based mainly on patients with lymphoma, whereas studies on breast cancer patients were few. Moreover, these studies were retrospective. Recently, a prospective study has recommended that deferred preemptive lamivudine could be a comparable alternative to the prophylactic strategy. However, it was not a randomized controlled study. In this study, it was examined the efficacy of the prophylactic strategy in hepatitis B s-antigen seropositive breast cancer patients during chemotherapy using a prospective, randomized controlled study. Two groups were studied. One group consisted of 21 patients who were treated with prophylactic lamivudine, the other group consisted of 21 patients who were not treated with prophylactic lamivudine. The results showed that the prophylactic lamivudine strategy significantly decreased the incidence of HBV reactivation (0 vs. 28.6%, P = 0.021). It was conclude that the prophylactic lamivudine strategy significantly reduces the incidence of HBV reactivation for hepatitis B s-antigen seropositive breast cancer undergoing chemotherapy.