Carcinomatous meningitis is a devastating metastatic complication of systemic carcinoma, which may occur insidiously, accompanied by a confusing spectrum of clinical symptoms and signs. In the absence of reliable diagnostic tumour markers, the diagnosis is established by the demonstration of malignant cells within the cerebrospinal fluid (CSF). Cytological techniques requiring skillful interpretation are occasionally negative in the presence of established disease, and when positive may indicate leptomeningeal malignancy of such advanced nature that effective palliation is difficult. Biochemical tumour marker technology offers the potential of reliable diagnosis in early disease states, prior to the appearance of exfoliated malignant cells. In a series of 100 patients, we assayed for an epithelial associated glycoprotein (HMFGI antigen) in CSF obtained at lumbar puncture. In 18 of 20 patients with carcinomatous meningitis, this high-molecular-weight glycoprotein was detectable in the CSF. The antigen was also present in 2 patients with neoplastic meningitis complicating lymphoma and medulloblastoma, but was not detected in the CSF of the remaining 78 patients.