We performed clinical analysis of 12 patients with renal cell carcinomas associated with tumor thrombosis in the inferior vena cava. Eleven cases were men, and one was a woman; their ages range from 48 to 76 years old with a mean of 58 years. Nine tumors were observed on the right side, the other 3 tumors were observed on the left side. In five cases, the distant metastases of the disease were noticed at the first visiting to our hospital. Lung metastases were found in five and bone or liver in each one. Chief complaints were macroscopic hematuria in 8 cases (67%), and were weight loss or general fatigue. The symptoms of obstruction of the inferior vena cava, such as venous dilatation of abdominal wall, edema of lower extremities and varicocele of the testes, were seen in 6 cases. The level of the tumor thrombosis was preoperatively determined by CT, echography, cavography or MRI. The level was near the right atrium in one, near the hepatic vein in 8 and near the renal vein in 3, although there was no case extending into the right atrium. Transperitoneal nephrectomy and thrombectomy in the inferior vena cava were performed in 9 cases. Surgery could not be performed in the other 3 patients of their poor general condition or severe heart disease. One patient died because of massive hemorrhage during the operation. The other complications were transient renal failure in 3 cases and postoperative bleeding in one case. In 4 patients without distant metastases or regional lymph nodes metastasis, two died of multiple metastasis of renal cell carcinomas and diabetic coma. The other two cases are alive without disease for 4 and 40 months after operation. For renal cell carcinoma extending into the inferior vena cava without metastasis, nephrectomy and thrombectomy should be performed using the extracorporeal circulation.