Abstract::In 44 out of 758 patients (5.8%) with non‐Hodgkin's lymphoma presenting between 1971 and 1982, the central nervous system (CNS) was involved. Patients with a diffuse histology had a 7.6% (34/449) incidence of CNS involvement compared to 3.9% (10/257) for patients with nodular lymphoma. In 63% of patients there was evidence of progressive systemic lymphoma at the time of diagnosis of CNS disease and in 23% CNS relapse occurred in clinical remission. Bone marrow was involved in 34% of patients at diagnosis and in 52% at some time prior to the onset of CNS complications. Cerebrospinal fluid cytology was positive in 63% and an elevated protein level was found in 95% of patients. The median length of survival of the 44 patients was only 3.2 months, but patients who responded to treatment of CNS lymphoma survived significantly longer than those who showed no response or progressed on therapy. Complete response to CNS treatment was achieved in five patients, of whom none relapsed in the CNS and two are long‐term disease‐free survivors. CNS prophylaxis appears justified for patients with lymphoblastic lymphoma, Burkitt's tumour, and diffuse undifferentiated lymphoma, who are at high risk of developing CNS complications. Patients with diffuse histiocytic, and diffuse poorly differentiated lymphocytic, lymphoma who have bone marrow involvement may also benefit from CNS prophy