Two patients with acquired immunodeficiency syndrome developed simultaneous Kaposi's sarcoma and bacillary (epithelioid) angiomatosis. The distinguishing clinical and histologic features of these two vascular proliferations associated with human immunodeficiency virus disease are described. The lesions of bacillary (epithelioid) angiomatosis contained bacteria, while the lesions of Kaposi's sarcoma did not. With erythromycin therapy, the lesions of bacillary (epithelioid) angiomatosis cleared, while those of Kaposi's persisted. Bacillary (epithelioid) angiomatosis, a treatable but potentially fatal opportunistic infection of human immunodeficiency virus disease, should be considered in the differential diagnosis of vascular lesions in immunosuppressed patients.