The human immunodeficiency virus HIV (formerly called HTLV- III/LAV) has now been clearly shown to be the etiologic agent of the acquired immunodeficiency syndrome (AIDS). HIV has also been implicated in the etiology of the progressive encephalopathy which occurs in children with AIDS. HIV is related to retroviruses of the lentivirus subfamily, which may remain inactive (latent) for long periods, or become active with cytopathic effect in target tissues, thus posing a severe problem for therapy in cases of central nervous system (CNS) involvement. We propose to assay HIV antigen expression in the cerebrospinal fluid (CSF) of children with HIV infection as a marker of viral activity, and to correlate this assay with clinical neurologic status as an index of disease progression. Importantly, the existence of a large number of intravenous drug abusing (IVDA) AIDS families among our patient population will allow us to perform longitudinal studies in a significant number of HIV-infected children.