CSF and serum of 80 HIV-seropositive patients in various stages of the disease were examined. Total as well as intrathecal IgG synthesis appeared to be frequently elevated in each diagnostic group. Abnormal CSF oligoclonal IgG fractions were significantly more frequent in asymptomatic patients. CSF pleocytosis was found with a lower percentage in neurological AIDS patients. In patients subdivided according to blood T helper cell count, the distribution of the abnormal CSF parameters confirms that the immunological response in the CNS is mediated by interaction with systemic immunity.