This study evaluated the relation between cerebrospinal fluid (CSF) mononuclear cells (MNC) and CSF HIV-1 RNA levels. HIV-1 RNA levels in plasma and CSF were analyzed by reverse transcription-polymerase chain reaction (RT-PCR) in 58 consecutive patients with neurologic symptoms and late HIV-1 infection. The majority of the patients had no central nervous system (CNS) complication (n = 36), 11 had AIDS dementia complex (ADC) and 11 had CNS opportunistic infection (CNS OI). CSF cell counts were analyzed using a method that also evaluated hypocellular CSF (i.e., from 0.1 x 10(6) cells/L). A strong correlation was found between CSF MNC and CD4+ lymphocyte counts in blood (r = 0.58; p < .0001). HIV-1 RNA was detected in all plasma samples and in 38 of 58 (66%) of the cell-free CSF samples. CSF HIV-1 RNA was less frequently detected in patients with hypocellular CSF than in patients with normocellular or pleocytic CSF (13 of 28 patients [46%] versus 10 of 14 patients [71%] versus 15 of 16 patients [94%], respectively). The levels of CSF HIV-1 RNA correlated with the CSF MNC count (r = 0.61; p < .0001). The correlation also remained strong within the clinical subgroups of CNS asymptomatic patients (r = 0.55; p < .001) and ADC patients (r = 0.79; p < .001), but not among CNS OI patients (r = 0.19). Patients with CNS OI were found to have higher CSF HIV-1 RNA levels than the patients without evidence of CNS complication. Thus, a close relation was found between CSF HIV-1 RNA levels and CSF MNC counts. These data support the hypothesis that a substantial part of the virus in the CSF of HIV-1-infected patients is locally produced by CSF MNC.