We measured the levels of interferon-γ (IFN-γ) and neopterin in the serum and cerebrospinal fluid of 121 human immunodeficiency virus-seropositive (HIV +) and 62 -seronegative (HIV −) individuals evaluated for neurologic disease. CSF levels of IFN-γ and serum and CSF levels of neopterin were higher in HIV + than in HIV− individuals. Patients with HIV− related meningitis and with opportunistic CNS infections had higher serum neopterin levels than HIV+ asymptomatic individuals. CSF levels of IFN-γ were slightly higher in CSF of HIV + individuals in all groups (0.31 ± 0.03 U/ml) than in HIV− individuals (0.12 ± 0.03). CSF levels of neopterin were similar in HIV+ asymptomatic individuals (6.9 ± 0.7 nmol/l) and HIV− individuals (5.9 ± 1.1), but were elevated in those HIV− infected individuals with neurologic disease, particularly patients with HIV− associated meningitis (72.1 ± 13.3 nmol/l), opportunistic CNS infections (36 ± 9.1), and inflammatory demyelinating polyneuropathies (32.4 ± 17.2). Levels of neopterin correlated positively with levels of soluble interleukin 2 receptor and soluble CD8, 2 additional indicators of immune activation. In the absence of neurologic disease, levels of IFN-γ and neopterin in both serum and CSF were stable for up to 4 years after seroconversion. These data suggest that increased CSF neopterin is associated with HIV-associated neurologic disease.