Papular and nodular skin lesions that clinically resembled Kaposi sarcoma, but histologically showed a distinct epithelioid haemangioma-like appearance, were noted in seven patients with the acquired immunodeficiency syndrome. Clusters of bacteria that had the structure of gram-negative rods were identified within each of the vascular proliferations by electron microscopy. The bacteria did not stain with the Brown-Brenn, acid-fast, or other histochemical stains for infectious organisms, but did stain with Warthin-Starry--ie, the staining profile was that described for the cat scratch disease (CSD) bacillus. Immunoperoxidase staining, using antisera raised in rabbits against cultured CSD bacillus, showed a positive reaction with the bacterium in all five cases tested. The two surviving patients have both given histories of having been scratched by a cat. In several patients, the vascular lesions regressed after therapy with antibiotics appropriate for CSD bacillus infection.