The polymerase chain reaction (PCR) wasused to detect human immunodeficiency virus type 1 (HIV-l) proviral sequences (gag and env) in nucleated cells from the cerebrospinal fluid (CSF) of 31 HIV-l-positive patients, and the results were compared with clinical and radiologic evidence of neurologic disease. Provirus was detected in 21 patients, of whom 20 had neurologic abnormalities. Provirus was not detected in another 6, all ofwhom were neurologically normal. No neurologic disease has developed in 4 of these 6 patients for whom 12.8 months of follow-up is available. PCR of CSF nucleated cells from HIV-positive patients provides early, rapid, direct evidence of neurologic involvement.