The polymerase chain reaction (PCR) and a virus culture technique were used to detect human immunodeficiency virus type 1 (HIV-1) DNA in cerebrospinal fluid (CSF) cells and infectious virus in cell-free CSF, respectively, of 28 HIV-1 seropositive homosexual men. Provirus was detected in 24 patients of whom 15 were also culture positive. One subject was virus culture positive but not PCR positive. Two asymptomatic HIV-1 seropositive persons and one individual with persistent generalized lymphadenopathy were negative by both techniques. All of four patients with overt neurological symptoms, but also 20 of 24 patients without such symptoms were PCR positive. The data indicate that viral replication is common, and that the vast majority of HIV-1-infected individuals harbor the virus DNA in CSF, during all stages of infection.