Patients with malignancies require chemotherapy and other immunosuppressive therapies for treatment. Because of this immunosuppression, in patients who have ever been exposed to hepatitis B it is possible for reactivation to occur. This reactivation can be fatal. Reactivation is particularly likely in patients who receive B cell–active agents such as rituximab. The occurrence of reactivation flares may also delay further chemotherapy, which can negatively affect the outcome of the underlying malignancy. Accordingly, it is important to screen patients for markers of hepatitis B and institute antiviral prophylaxis to prevent reactivation.