A76-year-old woman presented with diffuse large B-cell non-Hodgkin lymphoma (NHL), stage III A, low-intermediate International Prognostic Index. The disease involved the retroperitoneum, laterocervical, and supraclavicular regions. Bone marrow and other organ sites were negative. She received 5 of 6 planned cycles with rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone. One month after the fifth cycle, while in complete remission, she developed flaccid quadriplegia along with left hemiparesis, gaze palsy, dysphonia, and dysphagia. The clinical picture was suggestive of Guillain-Barre` syndrome but there was no response to high-dose intravenous immunoglobulins. Brain CT showed diffuse meningeal enhancement. The cerebrospinal fluid showed mononucleated cells 12 000/mm3, protein 151 mg/dL, and glucose 5 mg/dL. Microscopic examination of the fluid showed large mononucleated cells with multiple prominent nucleoli (see figure). The cells were positive for CD19 and CD20, as