BACKGROUND Although several reports have recently demonstrated that segmentectomy for small-sized N0 lung cancer leads to recurrence and survival rates equivalent to those associated with lobectomy, controversy regarding the postoperative functional advantage in the former over the latter still persists. The purpose of this study was to evaluate the degree of postoperative functional loss in patients undergoing segmentectomy or lobectomy for lung cancer. METHODS We analyzed patients able to tolerate lobectomy, who underwent radical segmentectomy (n = 38) or lobectomy (n = 45) for non-small-cell lung cancer. Functional testing included forced vital capacity, forced expiratory volume in 1 second, and anaerobic threshold measured preoperatively and at 2 and 6 months after surgery. RESULTS Preoperative function tests showed no differences between segmentectomy and lobectomy patients. A positive and significant correlation was found between the number of resected segments versus loss of forced vital capacity (r = 0.518, p < 0.0001 at 2 months; r = 0.604, p < 0.0001 at 6 months) and loss of forced expiratory volume in 1 second (r = 0.492, p < 0.0001 at 2 months; r = 0.512, p < 0.0001 at 6 months). The postoperative reduction of forced vital capacity (p = 0.0006) and forced expiratory volume in 1 second (p = 0.0007) was significantly less in the segmentectomy group; however, a marginally significant benefit was observed in this group for anaerobic threshold (p = 0.0616). CONCLUSIONS The extent of removed lung parenchyma directly affected that of postoperative functional loss even at 6 months after surgery, and segmentectomy offered significantly better functional preservation compared with lobectomy. These results indicate the importance of segmentectomy for early staged lung cancer.