The frequency of meningeal dissemination (MD) in primary CNS lymphoma (PCNSL), its prognostic impact, and optimal management have not been defined thus far. In 69 of 92 (75%) immunocompetent patients, primarily diagnosed with PCNSL at our institution between January 1994 and February 2007, cerebrospinal fluid was analyzed for MD. MD was found by cytomorphology in 7/63 (11%), by immunophenotyping in 1/32 (3%), and by PCR of the IgH CDR III region in 6/37 (16%). Neuroradiologic examination revealed MD in 3 of 69 patients (4%). Median event-free survival (EFS) of patients with MD diagnosed by any of the methods was 26 months, of those without MD 34.1 months (P = .24); median overall survival (OAS) of these two patients' groups was 45.5 and 42.5 months, respectively (P = .34). Patients with cytomorphologic proof of MD had a median EFS of 15.4 months and OAS of 18.5 months, those without MD 34.3 and 45 months (P = .018 and .017, respectively). We found a low frequency of MD despite the use of putatively sensitive diagnostic methods. No impact on outcome was seen for MD, diagnosed by any of the methods used; however, patients with cytomorphologic proof of MD had a significantly shorter median EFS and OAS.