It has been postulated that the retrovirus HTLV-III/LAV thought to be the etiologic agent of AIDS also infects the central nervous system and directly causes AIDS encephalopathy. Electron microscopical studies performed on brain sections from three patients with AIDS complicated by progressive encephalopathy revealed structures morphologically consistent with HTLV-III/LAV retrovirus particles. The particles were located principally within multinucleated giant cells but were also present in astrocytes. Many particles were also noted in the extracellular space.