Epstein-Barr virus (EBV) is constantly associated with AIDS-related primary lymphomas of the central nervous system (CNS). To assess whether EBV DNA in cerebrospinal fluid (CSF) could be used as a tumour marker, CSF samples that had been taken within 180 days before death from 85 patients with HIV infection and neurological disorders at necropsy were examined retrospectively by nested polymerase chain reaction (PCR) for EBV. Histologically evident primary CNS lymphomas were found in 17 patients, and EBV was shown in tissue by in-situ hybridisation in 16 of the 16 cases examined. All 17 patients with primary CNS lymphoma had EBV DNA in CSF. EBV DNA was found in CSF from 1 of 68 HIV-infected patients without histologically detectable lymphoma at necropsy. PCR for EBV DNA in CSF was 100% sensitive and 98.5% specific for AIDS-associated primary CNS lymphoma, and may be useful as a diagnostic tumour marker.