An aggressive variant of Kaposi's sarcoma (KS) has appeared in young homosexual men with evidence of systemic immunosuppression. The ultrastructure in biopsy specimens from 8 KS cases in young homosexual men has been compared with that in biopsy specimens from 4 KS cases in elderly heterosexuals and with that in biopsy specimens from 23 cases of benign vascular disorders of skin. In all cases of KS the small blood vessels lacked a prominent investment of pericytes and their processes, had a fragmented and often absent basal lamina, had frequent discontinuities in the endothelial lining, and had only a few small junctional densities between endothelial cells. Some clinically aggressive cases of KS also had necrosis of individual endothelial cells and had prominent cytoplasmic processes entrapping individual collagen fibers. The benign disorders lacked these features. These differences in the structure of the small vessels may be of diagnostic value in some early cases of KS. The loss of dendritic pericytes in blood capillaries in KS might relate to the telangiectasia which is a prominent feature of the early lesions of KS.