Kaposi's sarcoma is the most common tumor found in patients with the acquired immunodeficiency syndrome. This opportunistic neoplasm has characteristics of a sexually transmitted disease. Growth factors, cytokines, immune suppression, and interaction with infectious organisms all appear to play a role in the pathogenesis of this enigmatic disorder. The manifestations of Kaposi's sarcoma are protean, and lesions may appear at any time in the course of human immunodeficiency virus disease, remain localized and asymptomatic, or spread aggressively and cause morbidity. Treatment, which must be individualized, ranges from observation, local therapy with cosmetic makeup, and cryotherapy with liquid nitrogen or local intralesional injection of agents, to radiotherapy and systemic therapy with interferon-alpha and cytotoxic chemotherapy.