No definitive comparisons of surgical morbidity between segmentectomy and lobectomy for non–small cell lung cancer have been reported. We conducted a randomized controlled trial to confirm the noninferiority of segmentectomy to lobectomy in regard to prognosis (trial No. JCOG0802/WJOG4607L). Patients with invasive peripheral non–small cell lung cancer tumor of a diameter ≤2 cm were randomized to undergo either lobectomy or segmentectomy. The primary end point was overall survival. Here, we have focused on morbidity and mortality. Predictors of surgical morbidity were evaluated by the mode of surgery. Segmentectomy was categorized into simple and complex. Simple segmentectomy was defined as segmental resection of the right or left segment 6, left superior, or lingular segment. Complex segmentectomy was resection of the other segment. This trial is registered with the University Hospital Medical Information Network--Clinical Trial Registry (UMIN000002317). Between August 10, 2009, and October 21, 2014, 1106 patients (lobectomy n = 554 and segmentectomy n = 552) were enrolled. No mortality was noted. Complications (grade ≥ 2) occurred in 26.2% and 27.4% in the lobectomy and segmentectomy arms (P = .68), respectively. Fistula/pulmonary-lung (air leak) was detected in 21 (3.8%) and 36 (6.5%) patients in the lobectomy and segmentectomy arms (P = .04), respectively. Multivariable analysis revealed that predictors of pulmonary complications, including air leak and empyema (grade ≥ 2) were complex segmentectomy (vs lobectomy) (odds ratio, 2.07; 95% confidence interval, 1.11-3.88; P = .023), and > 20 pack-years of smoking (odds ratio, 2.61; 95% confidence interval, 1.14-5.97; P = .023). There was no difference in almost any postoperative measure of intraoperative and postoperative complication in segmentectomy and lobectomy patients, except more air leakage was observed in the segmentectomy arm. Segmentectomy will be a standard treatment if the superior pulmonary function and noninferiority in overall survival are confirmed. The 2 phase III trials, JCOG0802/WJOG4607L and JCOG0804/WJOG4507L, were based on JCOG0201. Display omitted