The measurement of the Haller index (HI) is not standardized, and HI does not consider the asymmetry of pectus excavatum. The aim of this study was to determine the most appropriate level for measuring the HI and to introduce the asymmetry index (AI) in order to respect the aspect of asymmetry. Preoperative computer tomography scans of 43 patients with pectus excavatum were retrospectively analyzed by measuring both the HI and the AI at 3 different levels: (I) sternomanubrial junction; (II) caudal end of corpus sterni; and (III) tip of the xiphoid. The control group comprised 33 patients without chest wall deformity and a HI < 3.25. For each patient HI and AI were calculated according to a standardized protocol. A McNemar test was used for statistical analyses. The sensitivity of the HI was highest when measured at level II (p < 0.388), and the AI exhibited the highest sensitivity at level I. When combining both indices, the sensitivity of assessing pectus excavatum significantly increased compared with the use of HI at level II alone (p < 0.002). The measurement of the HI at level II represents the most valid standardized parameter for assessment of the severity of pectus excavatum. Asymmetry, on the other hand, is best assessed by the AI measured at level I. The combination of the standardized HI and AI not only provides a much more accurate description of pectus excavatum, but also improves the comparability of pectus excavatum patients in general.