De novo malignant tumors have been observed throughout the world in 75 chronic survivors of organ transplantation, including 16 of our own patients. The incidence of tumors was approximately 80 times greater than in the average population in a comparable age range. Chronic uremia may have predisposed to the development of some of the tumors, but this has not yet been proved. It seems clear that the predominant etiology was chronic immunosuppression post-transplantation. Forty-four of the patients had epithelial tumors and in 31 the lesions were of mesenchymal origin. On the average the malignancies appeared 29 months after transplantation. Lymphomas showed an unusual-predilection for involvement of the central nervous system. Carcinomas of the skin, lip, and uterine cervix were successfully treated by conventional techniques. On the other hand, carcinomas of the thoracic or abdominal organs and mesenchymal tumors led or contributed to early death in most cases. For this reason drastic reduction or even discontinuance of immunosuppression should be considered in the management of these latter tumors.