Abstract OBJECTIVES Segmentectomies such as S1 + 2, S1 + 2+3 and S4 + 5 segmentectomy are used to treat patients with non-small-cell lung cancer (NSCLC) in the left upper lobe. However, the preservable lung volume and changes after such segmentectomies remain unknown. We compared the residual pulmonary function after thoracoscopic segmentectomy or lobectomy in the left upper lobe and examined the efficacy of S1 + 2 segmentectomy regarding postoperative pulmonary function. METHODS Patients with left upper lobe NSCLC who underwent thoracoscopic segmentectomy or lobectomy were included. Spirometry and computed tomography were performed before and 6 months after resection, and the ipsilateral preserved lobe volume was calculated using 3-dimensional computer tomography. The percentage of postoperative/preoperative forced expiratory volume in 1 s and actual/predicted regional forced expiratory volume in 1 s (preservation rate) in the residual lobe were compared. RESULTS Eighty-eight patients underwent lobectomy and 70 patients underwent segmentectomy (23 S1 + 2, 35 S1 + 2+3 and 12 S4 + 5 segmentectomies). The percentage of postoperative/preoperative forced expiratory volume in 1 s was 97 in S1 + 2, 82 in S1 + 2+3, 86 in S4 + 5 segmentectomy and 73 in left upper lobectomy, indicating that segmentectomy could be a meaningful approach to preserve pulmonary function. The preservation rate was 83% in S1 + 2 and 62% in S1 + 2+3 segmentectomy and was significantly higher in S1 + 2 than in S1 + 2+3 segmentectomy (P < 0.001). CONCLUSIONS Postoperative pulmonary function and the preservable lung volume of the residual lobe after thoracoscopic S1 + 2 segmentectomy were well-preserved among other segmentectomies and lobectomy. Thoracoscopic S1 + 2 segmentectomy is a good alternative for preserving postoperative function.