We report a case of renal cell carcinoma in whom preoperative interferon-alpha therapy was successful in reducing the primary lesion and the tumor thrombus extending into the inferior vena cava. A 67-year-old man was referred to our hospital because of his right renal mass. We made a diagnosis of a right renal tumor with tumor thrombus by imaging examinations. Because his performance status was 3, a radical operation was considered risky. So we started interferon-alpha therapy. Four months after the start of interferon-alpha therapy, the primary lesion and the tumor thrombus reduced in their size, and the clinical response was evaluated as partial response by the response criteria for urological cancer treatment. Because of improving the performance status and reducing the tumor size, radical nephrectomy and tumor thrombectomy could be performed. Pathological examinations revealed that viable renal cell carcinomas were found only in some small parts of the primary lesion, but not in the tumor thrombus. Postoperatively, no recurrence and metastasis of renal cell carcinoma of the patient have been detected in these two and a half years.