To verify how the components of the capillary wall are modified in the course of AIDS we studied the brain cortex from nine cases with AIDS. Cellular and extracellular components were delineated using antibodies for laminin and collagen IV for basement membranes and glial fibrillary acidic protein for astrocyte foot processes. We found a marked increase in reactivity for laminin in the basement membranes of capillary walls and hypertrophy and hyperplasia of astrocyte foot processes around vessels, when compared to control cortical tissue. We suggest that modifications of brain capillary wall may have a role in the pathogenesis of neurological disfunction in AIDS.