Breast cancer is a rapidly increasing problem in many developing countries, and cytotoxic chemotherapy is now an integral part of its management. In several developing countries, the carriage of hepatitis B virus (HBV) in cancer patients may be as high as 12%, and such patients are at risk of developing fatal HBV reactivation during chemotherapy. HBV reactivation is well recognized in patients with hematological malignancies, but limited data are available on patients with other, more common, cancers, such as breast cancer. Recent data have suggested that increased viral replication, an indication of HBV reactivation, may precede clinical hepatitis. In the absence of serial HBV DNA monitoring, HBV reactivation during chemotherapy may have been underestimated. In this prospective study, breast cancer patients who were hepatitis B surface antigen (HBsAg) seropositive were followed up during chemotherapy. The main objectives were to determine the incidence of HBV reactivation in breast cancer patients undergoing conventional chemotherapy; to investigate whether “serial HBV DNA monitoring” improves the accuracy of diagnosing HBV reactivation when compared with previous schema that only measured HBV DNA at the time of clinical hepatitis (“conventional monitoring”); and to assess the clinical consequences as a result of developing the condition. The secondary objective was to identify risk factors associated with this condition. Over an 18-month period, 41 patients were studied. Ten developed HBV reactivation by conventional monitoring criteria, but with serial HBV DNA monitoring, seven additional patients were diagnosed when increased HBV DNA levels were detected before, but not concomitant with, clinical hepatitis. Thus, a total of 17 patients (41%) developed HBV reactivation. Premature termination of chemotherapy or delay in treatment schedules occurred in 71% of the patients who developed viral reactivation, as compared with 33% in those who did not develop the condition (P = 0.019). No risk factors associated with the development of HBV reactivation could be identified. Serial monitoring of HBV DNA, in addition to liver function, increases the sensitivity of diagnosing of HBV reactivation, and helps explain some cases that would otherwise be labeled as “cryptogenic hepatitis,” for which concomitant HBV DNA measured at the time of hepatitis was undetectable. The present study highlights the importance of monitoring HBsAg-seropositive patients who are receiving chemotherapy for common solid tumors such as breast cancer. J. Med. Virol. 70:553–561, 2003. © 2003 Wiley-Liss, Inc.