Central nervous system (CNS) lymphoma was identified in 96 patients treated for non-Hodgkin's lymphoma at Memorial Sloan-Kettering Cancer Center between 1975 and 1981. During the same period, 68 other patients with non-Hodgkin's lymphoma but no CNS disease received prophylactic CNS chemotherapy. In the 156 total patients, the lymphomas were diffuse in 96 percent, and 67 percent were stage IV at diagnosis. CNS involvement was present at initial diagnosis in 27 percent, at relapse in 26 percent, and during the course of progressive systemic disease in 47 percent. CNS involvement was asymptomatic in 10 percent. Cytologic study of the cerebrospinal fluid was the most sensitive and specific laboratory test, but often (22 percent) more than one lumbar puncture was required to identify malignant cells. CNS lymphoma was treated in 85 patients, 46 by intracerebroventricular cannulae; 81 percent improved. Although median survival after the diagnosis of CNS disease was four months, there were seven long-term disease-free survivors and the CNS disease contributed to death in only 14 percent. In 52 percent of treated patients, there was no CNS lymphoma at autopsy. CNS prophylaxis was with methotrexate or cytosine arabinoside, usually by lumbar puncture; an intraventricular cannula was used in seven patients. Although this group of high-risk patients with non-Hodgkin's lymphoma had a high systemic response rate and the median projected survival was greater than five years, CNS lymphoma developed in eight patients (12 percent). In five, CNS lymphoma occurred as an apparently isolated relapse site. The role of CNS chemoprophylaxis in high-risk patients with non-Hodgkin's lymphoma is still uncertain.