Mantle cell lymphoma (MCL) is a rare type of non-Hodgkin's lymphoma which is thought to derive from the cuff of the lymphoid follicle. The histopathologic and immunophenotypic features of MCL are well described. The literature contains few cytopathologic studies of collected cases of MCL. Review of files from the National Institutes of Health from 1989 through June 1993 revealed a total of 24 positive specimens from eight patients with a diagnosis of MCL. The specimens consisted of ten pleural effusions, ten cerebrospinal fluids (CSF), and four fine-needle aspirations. CSF involvement was noted in 3 of 8 (37.5%) patients and was associated with disease progression. The cardinal morphologic features on air-dried, Diff-Quik-stained material are a monotonous population of relatively small atypical lymphoid cells with enlarged, frequently grooved nuclei, coarse chromatin and small nucleoli, scant cytoplasm, and an absence of large or "transformed" lymphoid cells. Immunocytochemistry is characterized by expression of one or more pan-B-cell markers, immunoglobulin light-chain restriction, and positivity for the pan-T-cell antigen CD5. When these morphologic and immunocytochemical characteristics are present, the specific diagnosis of MCL can be suggested on cytologic specimens.