The use of unilateral pulmonary artery occlusion (UPAO) test for the preoperative evaluation of pneumonectomy was reported in adult patients. On the contrary, in infants, no strategies have yet been recommended to predict hemodynamics after pneumonectomy, nor has use of the UPAO test been reported. We describe the first case of infant with abnormal pulmonary circulation in whom successful pneumonectomy was performed after preoperative evaluation using UPAO test. Right pneumonectomy was planned for an 8-month-old girl, because of decreased right pulmonary function, high risk of pneumothorax, and impaired left lung expansion due to overexpansion caused by severe left bronchial stenosis and bronchomalacia. However, she had also prolonged pulmonary hypertension and there was difficulty in accurate echocardiographic evaluation of its severity due to concomitant left pulmonary artery stenosis. Furthermore, contrast-enhanced computer tomography suggested a certain degree of right pulmonary venous flow, discordant with the result showing scarce right pulmonary flow in perfusion scintigraphy. Predicting postoperative hemodynamic changes was therefore considered difficult. To evaluate these concerns, we performed cardiac catheterization and UPAO test to simulate postoperative hemodynamics. Pulmonary arteriography showed decreased but significant right pulmonary arterial and venous flows. Measurements including pulmonary artery pressure and cardiac index showed no marked changes after occlusion. Based on UPAO test results, the operation was successfully performed and hemodynamics remained stable postoperatively. The UPAO test may be useful for infants with cardiopulmonary impairment to evaluate the tolerability of pneumonectomy.