In a review of 8724 de novo malignancies that occurred in 8191 organ allograft recipients sarcomas were 7.4% of cancers. Kaposi's sarcoma (KS) made up 5.7%, and other sarcomas (OS) 1.7% a much higher proportion than in the general population. KS was most common in Arab, black, Italian, Jewish, or Greek patients. In 60% of patients with KS the lesions were confined to the skin and/or oropharynx while 40% involved internal organs and/or lymph nodes. Complete remissions following various treatments occurred in 53% of the former group and 27% of the latter. In both groups 32% and 60% of remissions, respectively, occurred when the only treatment was reduction or cessation of immunosuppressive therapy. However, this treatment caused impaired function or allograft loss from rejection in 22 of 34 kidney recipients. Recurrent KS occurred in 5% of patients in remission when immunosuppressive therapy was resumed. Nine of 114 patients (8%) tested for human immunodeficiency virus were positive. Most OS arose in internal organs or soft tissues. The major types were fibrous histiocytoma (20 patients), leiomyosarcoma (15), fibrosarcoma (12), rhabdomyosarcoma (9), hemangiosarcoma (8), undifferentiated sarcoma (7) and mesothelioma (6). Several unusual features were noted. Remarkably, 10 of 105 (10%) sarcomas occurred adjacent to or in a renal (6) or hepatic (4) allograft. Leiomyosarcomas are rare in children, yet 5 of 15 (33%) occurred in pediatric patients. Three hemangiosarcomas occurred in forearms at sites of arteriovenous fistulas used for pretransplant hemodialysis access. One leiomyosarcoma and one fibrosarcoma occurred in previously irradiated areas. One patient with mesothelioma had a history of asbestos exposure and two others had possible exposure.