Late-onset grade 4 neutropenia occurred in 3 (5.6%) of 54 non-Hodgkin's lymphoma patients treated with rituximab between September 2001 and March 2004. Neutropenia appeared 5 to 25 weeks after administration of cytotoxic agents in combination with rituximab and recurred 4 and 17 weeks after the first onset in 2 patients. Five episodes occurred in a total of 332 cycles of rituximab therapy. Bone marrow findings at the time of late-onset neutropenia showed neutrophil maturation arrest with or without reversible myeloid dysplasia in 3 episodes and selective depletion of the myeloid series in 1 episode. Neither circulating immune complexes nor antineutrophil antibodies were detected during the 3 episodes that we evaluated. Bone marrow cells stained CD8- and CD57-. Late-onset neutropenia resolved 5 to 7 days after granulocyte colony-stimulating factor therapy was started. Further studies are needed to determine how rituximab functions and to identify appropriate countermeasures.