The authors investigated by immunofluorescence the thymic epithelium from AIDS patients, using several structural and functional markers of this tissue. In AIDS thymus, the epithelial reticular pattern was lost; and instead, clusters of round or spindle-shaped cells (assessed by their keratin content) were found. In addition, regions of epithelial necrosis, with loss of cell limits, were frequently observed. Thymulin content was decreased, and a partial loss of differentiation antigens of the thymic epithelium was also detected. The authors showed the presence of immunoglobulins and elements of the complement system directly bound to AIDS thymic epithelium, but not in any of the controls studied, including thymuses from patients who died after a long period of agony. These data suggest that the thymic epithelium can be included among the target tissues in AIDS, possibly by an indirect mechanism of autoimmune destruction.