Bacillary angiomatosis is a newly recognized multisystem opportunistic infection seen in the human immunodeficiency virus infection. The disease is marked by papular and nodular vascular skin lesions that clinically resemble Kaposi's sarcoma. Histologically, the lesions are different and show clusters of bacteria showing the structure of Gram negative bacilli staining with Warthin-Starry stain. Transmission electron microscopy shows that the organisms (1 to 2 microns) have a trilamellar wall structure. Treatment with oral erythromycin (2 to 3 g a day) for 2 to 4 weeks rapidly leads to resolution.