We report here the case of a 63-year-old man who had a diffuse large B-cell lymphoma associated with hemophagocytic syndrome (HPS). The lymphoma involved the spleen, bilateral adrenal glands, and paraaortic lymph nodes of the abdomen. In both the bone marrow and lymph nodes, hemophagocytosis was evident, and the laboratory findings were consistent with HPS. The lymphoma cells showed a CD4+, CD5+, CD10-, CD19+, CD20+, CD25+ and surface immunoglobulin microalpha/kappa+ immunophenotype. The patient was unintentionally treated with rituximab alone, resulting in complete resolution of the lymphomatous lesions as well as the features of HPS in response to the initial two doses of rituximab, although he developed gastric hemorrhage requiring vigorous resuscitation. After the completion of eight doses of rituximab, the patient remains free of disease with an excellent performance status.