AbstractAlthough two‐thirds of tumors occurring in the central nervous system (CNS) are primary neoplasms, only 10% of positive cerebrospinal fluid (CSF) specimens are from primary CNS tumors. In this study, we reviewed the cytologic findings of 21 positive CSF specimens from primary CNS tumors. A computer search identified 21 cases of positive CSF specimens from patients with primary CNS tumors from the archives. Follow‐up included review of medical charts and histologic correlation. The specimens were from 20 patients (9 females and 11 males). Their ages ranged from 6–83 yr, old with a mean of 30 yr. The cases included 9 medulloblastomas, 7 gliomas (3 glioblastoma multiformes, 2 anaplastic astrocytomas, and 2 ependymomas), 2 germinomas, 2 non‐Hodgkin's large B‐cell lymphomas, and 1 ganglioneurocytoma. Two cases were classified as suspicious and the remaining as positive for malignancy. Immunocytochemistry was employed in 3 cases to support the cytologic diagnosis. These cases included one large‐cell lymphoma (leukocyte‐common antigen‐positive), one germinoma (placental alkaline phosphatase‐positive), and the ganglioneurocytoma (neuron‐specific enolase‐ and synaptophysin‐positive). There were no false‐positive cases. Our results suggest that positive CSF cytology in patients with a primary CNS tumor is a reliable indicator of malignancy and reflects leptomeningeal involvement. The use of immunocytochemistry is helpful in confirming the cytologic impression in some cases. Diagn. Cytopathol. 2002;26:209–212. © 2002 Wiley‐Liss, Inc.