We performed a unilateral pulmonary arterial occlusion (UPAO) test for the preoperative evaluation of right ventricular functions as a loading test in patients undergoing a lung resection without cardiac complications preoperatively. We investigated the relationship between changes in right ventricular hemodynamic functions and postoperative cardiac complications, namely right heart failure or arrhythmia. To evaluate the right ventricular hemodynamic function test, we measured the mean pulmonary arterial pressure, cardiac index, right ventricular ejection fraction end-diastolic volume, and stroke volume before and during the UPAO test using the thermodilution method, and calculated the total pulmonary vascular resistance and right ventricular stroke work indexes. The incidence of postoperative cardiac complications was not related to the changes in the total pulmonary vascular resistance index. However, the postoperative cardiac complications were common in patients whose right ventricular end-diastolic volume index was increased by more than 20% during the UPAO test. These results suggest that the changes in the right ventricular end-diastolic volume index during the UPAO test can predict postoperative cardiac complications in patients undergoing a pulmonary resection.