Forty patients with positive CSF cytology for subarachnoid dissemination of neoplasms were examined by magnetic resonance (MR) imaging for the detection of intracranial or intraspinal CSF metastases. The MR evidence of cerebral leptomeningeal metastases was noted in 12 of 54 unenhanced (22.2%) and 7 of 20 (35%) gadolinium-enhanced studies. However, in only 2 of the 20 (10%) gadolinium-enhanced scans did the enhanced brain images alone demonstrate the presence of CSF seeding. Four of 29 (13.8%) unenhanced studies of the spine and 6 of 16 (37.5%) gadolinium-enhanced spine studies were positive for neoplastic deposits on the spinal nerves or cord. Magnetic resonance without and with gadolinium enhancement was most likely to be positive in studies of patients with a non-CNS primary malignancy (16/51 = 31.4%) and least accurate with lymphoma or leukemia (1/18 = 5.6%). Although gadolinium administration increases the ability of MR to detect leptomeningeal metastases (particularly in the spine), the overall sensitivity of unenhanced and enhanced MR examinations is low (19.3 and 36.1%, respectively) in patients with proven cytological evidence of neoplastic seeding.