The ultrastructural morphology of Kaposi’s sarcoma of the oral mucosa in patients with acquired immune deficiency syndrome (AIDS) was examined electron-microscopically. Neoplastic spindle cells and vascular endothelial tumor cells are frequently involved in the phagocytosis of extra vasated erythrocytes. Pericytes, fibroblasts, macrophages and other phagocytic cells of the tumor stroma also participate in this process. Erythrocytes are also phagocytized by some of the keratinocytes belonging to the intact surface epithelium covering the Kaposi’s sarcoma lesion. Both the phenomenon of erythrophagocytosis and the presence of tubuloreticular structures in the cytoplasm of Kaposi’s sarcoma cells in patients with AIDS seem to be prominent ultrastructural markers and thus of diagnostic value.