Cancer incidence in patients who undergo transplantation ranges from 4% to 18% (average, 6%). We have data on 7248 types of malignancy that developed in 6798 patients. The predominant tumors are lymphomas, skin and lip carcinomas, vulvar or perineal carcinomas, in situ uterine cervical carcinomas, and Kaposi's sarcoma. Tumors appear relatively early after transplantation. The earliest are Kaposi's sarcomas, which appear an average of 22 months after transplantation, whereas the latest are tumors that involve the vulva and perineum, which appear an average of 113 months after transplantation. Unusual features of the lymphomas include a high incidence of non-Hodgkin's lymphomas, frequent involvement of extranodal sites, marked predilection for the brain, and frequent allograft involvement by tumor. Lymphomas are much more common in heart or heart and lung recipients than in kidney recipients, and in pediatric patients compared with adults. Neoplastic regression after reduction or cessation of immunosuppressive therapy occurs in some patients with non-Hodgkin's lymphoma or Kaposi's sarcoma.