Of the 67 patients who were diagnosed with malignant histiocytosis at the Department of Medicine, Chulalongkorn Hospital, from January 1981 to May 1992, seven (10%) were found to have neurological manifestations. In five patients, neurologic diseases were the presenting clinical features. In the remaining two patients, the disease manifested at the time of relapse. Leptomeningeal infiltration, the most common pattern of involvement, was found in five patients. Involvement of the spinal cord was noted in three patients and cerebral parenchyma in two individuals. Two patients demonstrated peripheral neuropathy. Rapid clinical deterioration was observed in all patients after documentation of neurological diseases with a mean survival of 1.8 months. It is concluded that the frequency and pattern of neurological manifestations in patients with malignant histiocytosis are comparable to patients with non-Hodgkin's lymphoma. However, most patients with malignant histiocytosis manifested the neurological symptoms as an initial presentation while in non-Hodgkin's lymphoma, neurologic diseases usually manifest at the time of relapse or disease progression.