Prevalence of HIV-Ag in both serum and CSF has been determined in 19 HIV infected patients, including 7 patients without any symptoms or only generalized lymphadenopathy, 5 patients with ARC and 7 patients with AIDS. The results have been correlated with clinically evident neurological disorders. HIV-Ag have been detected in 9 out of 12 patients with ARC (AIDS Related Complex) and AIDS. In 8 of them neurological disorders have been present. Out of the remaining 7 patients in only one HIV-Ag has been detected in CSF (p < 025). No correlation between the presence of HIV antigen in CSF and serum has been noted.