Thirty-five primary non-Hodgkin's lymphomas (NHL) of the central nervous system (CNS) were examined in a retrospective study and classified according to the Kiel classification. Diagnosis was made on surgical biopsies in 28 cases and on autopsy specimens in seven cases. The tumors were predominantly located in the frontal lobe and were unifocal in all but two cases. Patients ranged in age from seven to 74 years, with a male-female ratio of 1.3:1. The lymphomas were probably all of B cell type. In contrast to NHLs of the lymph nodes, the NHLs of the CNS we encountered were more often high grade (immunoblastic, lymphoblastic) than low grade malignant. All but one low grade malignant NHL found in our study were lymphoplasmacytoid (LP) immunocytomas.