Oncological complications were studied in 420 renal transplantations performed in 389 patients. Seven patients with a history of neoplasia were transplanted, and 2 tumours were transmitted by the graft. However, the main problem was that of de novo tumours induced by immunosuppression: 26 patients developed such tumours which resulted in death for 7 of them. The risk of tumour seems to be relatively selective. Seven lymphoproliferative syndromes, 6 Kaposi's sarcomas and 4 spinocellular epitheliomas were observed and together accounted for 65.3 percent of de novo tumours. Excessive immunosuppression could be blamed in only one third of the patients. On the other hand, some geographical and/or ethnic factors seemed to facilitate tumour development, and infections with viruses of the herpes group were found in 76 percent of these subjects.