Segmentectomy includes numerous kinds of procedures that may result in decreased postoperative pulmonary function. This causes controversy regarding the functional advantage of segmentectomy over lobectomy. To clarify the difference between the procedures, systemic and regional pulmonary functions of the resected segments must be examined.Pulmonary function tests and lung perfusion single-photon emission computed tomography (SPECT) were prospectively conducted before and after segmentectomy in 117 patients who were divided into groups based on resection of <2 segments (n = 83), ≥2 segments (n = 20), and left upper division (LUD) (n = 14). Left upper lobectomy (n = 13) was used as a control for the LUD group. Forced expiratory volume in 1 second (FEV1) of segment and lobe were measured from a fusion image of SPECT and computed tomography.Percentage of postoperative/preoperative pulmonary function was the highest in the <2 segments group (97% ± 10%), which was followed by the ≥2 segments group (90% ± 9%), LUD group (84% ± 7%), and left upper lobectomy group (83% ± 7%), and the differences between the segmentectomy groups were significant (P < .001-.03), although there was no difference between the LUD and lobectomy groups. Whereas actual FEV1 of preserved lobes were significantly lower than the predicted value in all segmentectomy groups (P < .001), the percentage of actual/predicted value in the LUD group (43% ± 19%) was significantly lower than those in the <2 (72% ± 23%) and ≥2 segments (68% ± 30%) groups (P < .001 and P = .02, respectively).Segmentectomy decreased the pulmonary function with increasing number of resected segments. LUD segmentectomy decreased both systemic and lobar function significantly due to not only large resection, but also marked depression of the preserved lobe, resulting in similar decrease as lobectomy.