A unique case of a metastasising choriocarcinoma, inadvertently transplanted to a man from a female cadaver kidney is reported. When the kidney was removed six days after transplantation, arterial blood vessel infiltration by chorio-carcinoma cells and high levels of human chorionic gonadotropin (HCG) in the serum of the recipient indicated an haematogenous dissemination of viable neoplastic cells. The immunosuppressive therapy was discontinued after removal of the graft and the HCG levels in the recipient gradually decreased over a periode of eight weeks, indicating a slow immunologic rejection of the tumor cells. The recipient committed suicide seven months after the transplantation had failed. No metastases were found at a legal autopsy. It seems advisable, whenever there is evidence that neoplastic cells might have been transfered by a homograft, to remove the graft and discontinue the immunosuppressive therapy. Neoplastic cells already disseminated can still be eliminated when the immunologic system is intact and not suppressed.