Six patients with malignant retroperitoneal tumor extending into the inferior vena cava (VCI) were surgically treated with use of a normothermic extracorporeal circulation (ECC). Origin of malignant tumor was renal cancer in four, adrenal cancer in one and seminoma in one. Four patients excluding two of renal cancer had metastasis to the lung or bone preoperatively. One of renal cancer with lung metastasis and invasion to the colon died six months after surgery. The remaining five patients have been alive and well with follow up of 1 year to 7 years (mean 3 years 3 months). Cytological examination of an arterial filter of ECC and intra-circuit blood was made and negative in two patients. Long term results and cytology revealed a rare chance of intraoperative dissemination of malignant cells. These results suggested feasibility of surgery using normothermic ECC for the treatment of malignant retroperitoneal tumor with extension into VCI.