Reactivation of hepatitis B virus replication in patients receiving cytotoxic therapy. Report of a prospective study. One hundred Chinese patients who received induction cytotoxic therapy for malignant lymphoma were prospectively studied to determine the incidence, morbidity, mortality, and predisposing factors for reactivation of hepatitis B virus replication during cytotoxic therapy. In 18 (67%) hepatitis B surface antigen-positive and 10 (14%) hepatitis B surface antigen-negative patients, hepatitis developed during cytotoxic therapy (P less than 0.0001). Hepatitis could be attributed to exacerbation or reactivation of chronic hepatitis B in 13 (72%) hepatitis B surface antigen-positive patients but in only 2 (20%) hepatitis B surface antigen-negative patients (P less than 0.0001). Sudden increase or reactivation of hepatitis B virus replication gave rise to icteric hepatitis, nonfatal hepatic failure, and death in 22.3%, 3.7%, and 3.7% of patients who were positive for hepatitis B surface antigen; in 2%, 2%, and 0% of those positive for hepatitis B antibodies; and in none of those who were seronegative. Among the hepatitis B surface antigen-positive patients, male sex was the only factor that was associated with an increased risk of reactivation of hepatitis B virus replication. We recommend that hepatitis B surface antigen-positive patients with malignancies receiving cytotoxic therapy be closely monitored. 