Ninety-seven Japanese patients with so-called primary non-Hodgkin's lymphoma of the central nervous system (CNS-NHL), unrelated to the acquired immunodeficiency syndrome (AIDS) or organ transplantation, were reviewed. The patients' ages ranged from l to 87 years (median: 58 years) with a male to female ratio of I .77: I. The most frequent past histories were acute appendicitis (appendectomy), head injury, uveitis or iritis, and gastritis or gastric ulcer. These patients presented with symptoms suggesting an expanding intracranial lesion with no signs of extracranial lymphomatous disease. Combined computed tomographic scans, angiography, and findings at surgery or autopsy showed that the cerebrum was the commonest site of involvement, 87% of all cases, with the frontal to temporal region being the most commonly involved. Histologically, the diffuse large-cell type was most frequent and 26% of lymphomas were of high-grade malignancy as defined by the Working Formulation. The reported frequency of high-grade CNS-NHLs in AIDS patients in the United States is much higher (over 60%). Immunohistochemistry on paraffin-embedded sections revealed a 6-cell nature of the present series of tumors. In 16% of the cases, large numbers of small lymphoid cells with a positive reaction predominantly for anti-T lymphocyte antibodies surrounded the tumors or aggregated around the capillaries. The tumors which were infiltrated by small lymphoid cells showed more favorable prognosis than those which were not, suggesting a host reaction to tumor growth in these patients.