The true incidence of leptomeningeal tumor in primary central nervous system lymphoma is unknown. We studied prospectively the cerebrospinal fluid profile of 96 patients without acquired immunodeficiency syndrome but with primary central nervous system lymphoma at diagnosis, at completion of treatment, and at recurrence. Magnetic resonance images and pathology slides were examined for evidence of leptomeningeal tumor. Leptomeningeal tumor was diagnosed by (1) positive findings on cerebrospinal fluid cytology, (2) leptomeningeal or subependymal enhancement on magnetic resonance imaging, or (3) pathological evidence of leptomeningeal tumor. We recorded whether treatment directed against the leptomeninges was given. Cerebrospinal fluid was examined in 86 of 96 patients at diagnosis and 29 of 42 at recurrence. The incidence of leptomeningeal tumor was 42% at diagnosis and 41% at recurrence. Only elevated levels of protein and lactate dehydrogenase isoenzyme-5 were significantly associated with leptomeningeal tumor (p = 0.012, p = 0.016, respectively). Treatment against the leptomeninges was significantly associated with the probability of achieving a complete response and a longer freedom from relapse. Patients older than 50 years had a worse disease-specific survival but a similar probability of responding to therapy as younger patients. Our data show that leptomeningeal tumor in primary central nervous system lymphoma is more prevalent than originally thought, and indicate the need for therapy inclusive of the leptomeninges in all patients.