Aim To describe a new finding in patients with pectus excavatum – left lower lobe anterior basal segment hyperinflation [LABSH]. A secondary objective is to determine the frequency of the new finding and association with pectus excavatum severity. Material and methods The study population included 52 children who underwent preoperative computed tomography (CT) for the evaluation of pectus excavatum. A control group of 50 children was obtained after evaluating 137 CT examinations performed for other reasons. Patient age in both groups ranged from 12 to 20 years. The Haller index was calculated for all patients. LABSH was evaluated by visual inspection of lung windows. The difference in mean radiodensity measurements in regions of interest in the left and right anterior basal segments [ΔHU] was calculated. Spirometry was performed in 44 of the patients and the results were compared to Haller index severity and the presence of LABSH. Echocardiography reports were available for 50 children in the pectus excavatum group. Results LABSH was identified by visual inspection in 15 patients [29%] and was significantly associated with a Haller index >4.0 (p=0.001). ΔHU for the patients with LABSH was 90.2 HU (standard deviation [SD]=37.7) and for the non-hyperinflated group −5.51 (SD=44.63), which was significant (p<0.0001). There was a significant association of LABSH with the pectus excavatum group as compared to the control group. The difference in mean Haller index for children with normal spirometry (4.4, SD=2.7) was not significantly different (p=0.9899) than for children with obstructive disease (4.5, SD=1). There was mild cardiac compression on two echocardiograms. Conclusion LABSH is a new sign associated with pectus excavatum. The sign suggests segmental bronchial compression caused by chest deformity results in segmental air trapping.