Three homosexual men with acquired immunodeficiency syndrome (AIDS) were referred to Hvidovre Hospital, Copenhagen, during 1981. Biopsies from lymph nodes (case 2), skin (case 1 and 3), oral mucosal membrane (case 1) and lung (case 3) revealed Kaposi Sarcoma (KS). Furthermore, KS was found at autopsy in several other organs in case 3. While the tumor morphology of the lymph nodes, the oral mucosal membrane, and the internal organs was identical to that of classical KS, the skin lesions appeared different. Macroscopically, the skin lesions in case 1 and 3 were alike, but microscopically, the lesions in the two cases differed from each other. In both cases small tumor infiltrates were dispersed in the dermis with interjacent preserved dermal tissue. In case 1 the tumor infiltrates were composed of different vascular structures including irregular slits formed by dermal collagen fibres lined with long slim tumor cells. In case 2 the infiltrates were mainly composed of solid collections of pale tumor cells with faintly stained nuclei. The difficulties in establishing a diagnosis of KS on skin biopsies from patients with AIDS are emphasized.