Rituximab is a monoclonal antibody that targets B-lymphocytes, and it is widely used to treat non-Hodgkin's lymphoma. However, its use has been implicated in HBV reactivation that's related with the immunosuppressive effects of rituximab. Although the majority of reactivations occur in hepatitis B carriers, a few cases of reactivation have been reported in HBsAg negative patients. However, reactivation in an HBsAg negative/HBsAb positive patient after rituximab treatment has never been reported in Korea. We present here an HBsAg-negative/HBsAb-positive 66-year-old female who displayed reactivation following rituximab plus CHOP chemotherapy for diffuse large B-cell lymphoma. While she was negative for HBsAg at diagnosis, her viral status was changed at the time of relapse as follows: HBsAg positive, HBsAb negative, HBeAg positive, HBeAb negative and an HBV DNA level of 1165 pg/ml. Our observation suggests that we should monitor for HBV reactivation during rituximab treatment when prior HBV infection or occult infection is suspected, and even in the HBsAg negative/HBsAb positive cases.