A new indicator to assess trunk asymmetry in scoliosis patients is introduced. The medio-lateral length difference index at the axilla and the waist, and off balance index at C7 were defined as Frontal Asymmetry Index (FAI-C7, FAI-A, FAI-T). The height difference at the shoulder, the axilla and the waist were defined as Height Difference Index (HDI-S, HDI-A, HDI-T). The total sum of these six indices was defined as Posterior Trunk Symmetry Index (POTSI). The average intra-observer and inter-observer error of POTSI were 5.5 and 6.4 respectively. Slight trunk asymmetries were detected in normal children, whereas POTSI were significantly larger in scoliosis patients than those in normal children. This measurement of trunk asymmetry defined as POTSI is valid for the clinical assessment of scoliosis and to evaluate objectively the effect of surgery on trunk deformity.