Astroglial changes in the cerebral cortex of AIDS brains were analysed by means of morphometry. Astrocytes with and without immunoreactivity for glial acidic protein (GFAP) were counted and their size was measured. In the two investigated cortical areas (frontal and parietal), a similar reaction pattern of astroglia was observed. The total number of astrocytes (i.e. GFAP-positive and GFAP-negative astrocytes) did not differ between control and AIDS brains. However, the number of GFAP-positive astrocytes was significantly increased in AIDS brains, while the number of GFAP-negative cells was significantly reduced. Nuclear size of GFAP-negative and GFAP-positive astrocytes was significantly increased. The reaction pattern of cortical astrocytes in AIDS seems to be characterized by GFAP production in protoplasmic astroglia as well as by hypertrophy of all astrocytes.