We developed a new method that provides a wide retroperitoneal irradiation field for patients with cancer of the pancreas in whom a long invaded portal vein is resected during intraoperative radiotherapy. This method involves the use of a venous bypass with a heparinized catheter that does not require systemic anticoagulation or a pump. Anastomosis is completed after intraoperative radiotherapy. Mean (+/- SD) duration of the venous bypass was 3.0 +/- 1.5 hours (range, 2.0 to 6.6 hours), during which no abnormalities of systemic blood pressure, heart rate, and electrocardiographic activity and no signs of congestion of the intestines were observed. Eleven patients were treated with this method, and no complications ascribed to intraoperative radiotherapy or venous bypass were found. We recommend this method as safe and simple for expanding the irradiation field for intraoperative radiotherapy in patients with pancreatic cancer in whom the portal vein is resected.