There is a markedly increased incidence of de novo malignant tumors in patients who undergo organ transplantation. We have reviewed the neoplasms occurring in our own series of cardiac and renal allograft recipients, and compared our findings with neoplasia in the general population and at transplantation centers throughout the world. Skin cancer and lymphomas are the most frequently encountered neoplasms in transplant recipients, accounting for 7(70%) of 10 neoplasms at our own institute and for 1050(55%) of 1897 neoplasms worldwide. The physiopathological epidemiology of these malignant tumors is discussed, particularly with regard to the site distribution and pathologic types of tumors that arise de novo after organ transplantation, and to the probable etiologies and mechanisms involved in their development. © 1985.