Cutaneous vascular proliferations that clinically or pathologically resemble Kaposi's sarcoma, pyogenic granuloma, or histiocytoid (epithelioid) hemangioma may occur in patients with human immunodeficiency virus infection. These lesions, which respond well to antibiotic therapy, harbor bacilli similar to the agent of cat scratch disease. We evaluated 21 biopsy specimens from 13 patients with this condition, which we have called "bacillary angiomatosis." The architecture resembled that of lobular capillary hemangioma (pyogenic granuloma), but the endothelial cells were often larger, polygonal, and sometimes markedly atypical. The presence of neutrophils, leukocytoclastic debris, and granular material (bacteria), and the absence of either spindled cells, bizarrely shaped vascular channels, or hyaline globules help to distinguish bacillary angiomatosis from Kaposi's sarcoma. By electron microscopy, the protuberant endothelial cells were different from those of histiocytoid hemangiomas in that aggregates of intermediate filaments were absent, while numerous Weibel-Palade bodies were present. The immunophenotype of the endothelial cells was distinct from that of Kaposi's sarcoma; almost all cells showed both Factor VIII RAg and Ulex europaeus lectin positivity. Enzyme histochemistry also showed a pattern distinct from Kaposi's sarcoma. Bacillary angiomatosis presents a unique constellation of clinical and microscopic findings. It is important to be aware of these characteristics, because these lesions are easily treatable with antibiotic therapy.