In subjects with scoliosis the thoracic cage deformity is a complex 3D phenomenon. There is a deficiency of simple clinical methods of thorax shape evaluation. The study aimed to introduce and assess an anthropometric technique measuring transverse plane deformity of the thorax in patients with idiopathic scoliosis. Thirty scoliotic girls, aged 14.4±1.5 years, thoracic scoliosis type Lenke 1, mean Cobb 54.1±24.7°, and 30 healthy volunteers matched for sex and age were examined. Using a Martin anthropometric caliper the length of the long and the short horizontal axes of the thorax were measured at the level of the xiphoid process (upper index) and of the costal arch (lower index), both on maximum inspiration and expiration. Asymmetry index, defined as difference of the length of the long and the short axes expressed as the percentage of the short one, was calculated. The upper asymmetry index in the study group was 35.2±18.6 (inspiration) while in the control group it was 13.6±13.6, difference significant, p<0.001. The lower asymmetry index in the study group was 26.2±12.9 (inspiration) while in the control group it was 12.5±11.7, difference significant, p<0.001. In conclusion, thorax asymmetry index revealed significantly higher values in scoliotic patients. Asymmetry of respiratory movements could be measured. This simple technique may be used as a helpful tool for clinicians.