Morphologically atypical histiocytes were observed in the cerebrospinal fluid (CSF) of five patients with histiocytic proliferative disorders, including one each classified as Letterer-Siwe disease and Hand-Schüller-Christian disease and three as malignant histiocytosis. In all of these patients except the one with Letterer-Siwe disease, neurologic examination, CSF leukocyte count and protein and glucose concentrations were normal. Necropsy studies in the three fatal cases (one patient with Letterer-Siwe disease and two with malignant histiocytosis) demonstrated leptomeningeal involvement by atypical histiocytes in all three and brain involvement in the patient with Letterer-Siwe disease. These observations suggest that cytologic examination of CSF is warranted in patients with histiocytic proliferative disorders and may be reliable in documenting central nervous system, particularly leptomeningeal, involvement. The atypicality of the histiocytes in the CSF, however, was not indicative of the classification of the histiocytic proliferative disorder. Because the patient with Hand-Schüller-Christian disease continues to have a normal neurologic examination seven years after abnormal histiocytes were initially detected in his CSF, we believe that the clinical condition of the patient and classification of the histiocytic proliferative disorder are of primary importance for initiating aggressive therapy directed at the central nervous system.