OBJECTIVE The division of inferior pulmonary ligament (IPL) during upper lobectomy (UL) was believed to be mandatory to dilate the remaining lung sufficiently. However, the benefits, especially postoperative pulmonary function, remain controversial. This study aimed to evaluate whether IPL division leads to pulmonary dysfunction. METHODS This retrospective study included 213 patients who underwent UL between 2005 and 2018. They were categorized into an IPL division group (D group, n = 106) and a preservation group (P group, n = 107). Postoperative dead space at the lung apex, pulmonary function and complications were assessed using chest X-rays and spirometry. Changes in bronchial angle, cross-sectional area and circumference of the narrowed bronchus on the excised side were measured on three-dimensional computed tomography. RESULTS There was no significant difference in the postoperative complication rate, the dead space area, forced vital capacity (FVC), or forced expiratory volume in 1 s (FEV1) between the 2 groups after right UL (FVC; P = 0.838, FEV1; P = 0.693). By contrast, after left UL pulmonary function was significantly better in the P than in the D group (FVC; P = 0.038, FEV1; P = 0.027). Changes in bronchial angle did not significantly differ between the 2 groups. The narrowed bronchus's cross-sectional area (P = 0.021) and circumference (P = 0.009) were significantly smaller in the D group than in the P group after left UL. CONCLUSIONS IPL division during left UL caused postoperative pulmonary dysfunction and airflow limitation due to bronchial kinking. IPL preservation may have a beneficial impact on postoperative pulmonary function.