Thirty-one primary human renal carcinomas were transplanted into athymic mice of which ten produced tumors in the mouse host. Only tumors with a nuclear grade of 3 or 4 were successfully transplanted. The nuclear grades of the human tumor and transplant were similar; however, the cellular histology often varied. Patient prognosis appeared to be inversely related to successful tumor transplantation. In the transplant group, the 1-year survival was 30% in contrast to a 1-year survival of 83% among patients with renal cancers of similar stage and grade which did not produce tumors in the mice.