Diagnosis of central nervous system (CNS) non-Hodgkin's lymphomas may be difficult despite the use of sophisticated scans and routine cytologic methods. The use of an immunoalkaline phosphatase technique to examine cerebrospinal fluid (CSF) containing many mononuclear cells is described. Monoclonal proliferations of B-lymphocytes were demonstrated in six patients with neurologic abnormalities, whose clinical findings and subsequent clinical courses were those of lymphoma. The diagnosis of CNS lymphoma could not be made, despite multiple diagnostic procedures, until the immunocytochemical studies were performed. In three other patients, a lymphoproliferative disorder was suspected; however, examination of CSF showed many T-lymphocytes but no monoclonal B-lymphocytes, consistent with a reactive lymphocytosis. The subsequent clinical courses of these patients have shown no evidence of CNS lymphoma. Immunocytochemical studies of CSF lymphocytes are useful in differentiating benign from malignant proliferations.