Light-microscopic, immunohistochemical, and ultra-structural studies were performed on biopsy material from 15 young homosexual men with AIDS-associated mucocutaneous Kaposi's sarcoma; 19 Kaposi's sarcoma lesions in different developmental stages were investigated. These lesions showed multicentrically arising and proliferating vascular endothelia forming thick-walled and thin-walled capillaries and larger vessels, as well as spindle-shaped cells forming fascicles and bundles around them. Different amounts and organization of these two major cellular components were found in all stages of evolution of Kaposi's sarcoma lesions. Immunohistochemical and electron-microscopic techniques suggested that the spindle-shaped cells were of pericyte origin in different stages of maturation or, more rarely, lymphatic endotheliocytes. The skin lesions of AIDS-associated Kaposi's sarcoma occurred as a result of multi-centric angioneoplasia of rather slow progression, together with the proliferation of pericyte-like mesen-chymal cells, possibly representing a stromal reaction to the vascular proliferation. Both blood and lymphatic vessels seemed involved in this process. In early stages, scattered lymphocytic infiltration was an additional feature. Mitotic figures and cytologic atypia were not seen more frequently in early AIDS-associated Kaposi's sarcoma than in proliferating granulation tissue.