Rectourethral fistula caused by Kaposi's sarcoma. A 35-year-old man with the acquired immunodeficiency syndrome-related complex was evaluated for a persistent urethral discharge, pneumaturia and watery diarrhea. Radiographic and endoscopic procedures established the diagnosis of a rectourethral fistula. Perineal exploration and excision of the fistula revealed the pathological diagnosis of Kaposi's sarcoma. The differential diagnosis of an acquired rectourethral fistula and the significance of Kaposi's sarcoma are discussed. 