We encountered a 53-year-old man with general fatigue. Bone marrow investigations revealed an infiltration of CD20+CD5+CD23- cells and the presence of cyclin D1 lymphoid cells, leading to a diagnosis of mantle cell lymphoma, clinical stage IV. The first 2 lines of chemotherapy, CyclOBEAP (cyclophosphamide, vincristine, bleomycin, etoposide, doxorubicin, and prednisolone) and fludarabine-cyclophosphamide, produced only a transient decrease in serum lactic dehydrogenase levels, without a clinical remission. Because of the persistence of bone marrow hypoplasia, monotherapy with 375 mg/m2 rituximab was administered. The pancytopenia gradually improved, and a complete remission was obtained after 4 cycles of rituximab. The patient remains in complete remission 21 months after the third rituximab therapy for maintenance.