De novo cancers occurred after transplantation in 8,008 organ allograft recipients, who developed 8,531 different types of malignancy. Three hundred twenty-four liver recipients developed 329 cancers. There were striking differences in the patterns of neoplasms observed when these were compared with 7,200 tumors that occurred in renal allograft recipients. Lymphomas were much more common in liver allograft recipients (57% v 12% of all tumors), whereas skin cancers (39% v 15%), carcinomas of the cervix (4% v 1%), renal tumors (4% v 1%), and vulvar carcinomas (3% v 0.6%) were more common in renal allograft recipients. The high incidence of lymphomas is related partly to the more intense immunosuppressive therapy administered to hepatic allograft recipients and partly to the large percentage of pediatric patients among them. The intense immunosuppression also accounts for the much shorter induction times of lymphomas (mean, 15 v 46 months; P < .001) and nonlymphomatous tumors (mean, 27 v 72; P < .001) in liver compared with kidney recipients. The longer follow-up of renal recipients probably accounts for the higher incidence of the other tumors that tend to appear relatively late after transplantation. A remarkable feature was the high incidence of allograft involvement by lymphoma (44%). Complete remissions after treatment occurred in 11 of 28 patients in whom the lymphoma was confined to the allograft. A few tumors in liver recipients were related to the underlying disease for which transplantation was performed: hepatomas in patients who underwent transplantation for hepatitis B cirrhosis and colon carcinomas or cholangiocarcinomas in patients who underwent transplantation for chronic ulcerative colitis with sclerosing cholangitis. A surprising finding was the development of four leiomyosarcomas, three occurring in the allograft itself, in pediatric liver recipients.