The records of 24 patients with renal cell carcinoma involving the inferior vena cava who were free of metastatic disease at presentation were reviewed retrospectively. The over-all 2-year survival for the group was 45.8 per cent, with a mean survival of 38.9 months. When the group was analyzed according to the level of extension of the vena caval thrombus marked differences in the rate of survival and of incidence of local progression of disease were found. The 10 patients with an infrahepatic vena caval thrombus had a 2-year survival rate of 80 per cent and a mean survival of 61.4 months. Two patients (20 per cent) had extension of tumor into the perinephric fat and none had involvement of the regional lymph nodes. The 14 patients with a vena caval thrombus extending to the level of the hepatic veins or beyond had a 2-year survival rate of 21 per cent and a mean survival of 22.9 months. Tumor was present in the regional lymph nodes and/or perinephric fat in 9 of these patients (64 per cent). These results suggest that the level of vena caval involvement by tumor thrombus in patients with renal cell carcinoma has prognostic significance.