Meningeal involvement (MI) by non-Hodgkin's lymphoma (NHL) was seen in 38/602 patients (6.3%). In relation to histologic subtype the frequency of MI was: Follicular small cleaved and mixed cell 2/128 (1.6%), small lymphocytic and diffuse small cleaved cell 2/83 (2.4%), large cell and immunoblastic 13/295 (4.5%), small noncleaved cell 6/31 (19%), lymphoblastic 15/66 (23%). Risk factors that predict for MI were, besides histologic subtype, age under 40 yr, clinical stage IV, site of involvement (bone marrow, bone, skin gastrointestinal tract), and a poor response to initial therapy. In a Cox multivariate model encompassing the intermediate and high grade malignancy groups of the Working Formulation (WF), the 3 most important risk factors were histology, age, and stage. The estimated 1-yr probability of MI for combinations of the 3 risk factors was: 3 risk factors (61%), 2 risk factors (15-28%), 1 risk factor (4-8%), 0 risk factor (1.5%). At the diagnosis of MI, 84% of the patients had evidence of advanced systemic NHL, and the median survival after MI was 10 wk. CNS prophylaxis with whole-brain irradiation and intrathecal chemotherapy can only be recommended in patients with 2 or 3 risk factors. Improvement of the systemic chemotherapy might be the most important factor for prevention of MI in NHL.