The Haller Index (HI) has become standard for determining the severity of pectus excavatum. We compared patterns of cardiopulmonary dysfunction and their relationship with HI in patients with pectus excavatum.We performed cardiopulmonary exercise testing and chest computed tomography scans on 90 patients with pectus excavatum deformities at a regional pediatric hospital.The median HI was 4.9 in patients with combined dysfunction, 4.4 in patients with isolated pulmonary dysfunction, 3.6 in patients with isolated cardiac dysfunction, and 3.4 in patients with normal function. HI varied significantly by disease group (P < .009). HI was significantly lower in patients with normal forced vital capacity than with abnormal forced vital capacity (P = .001). However, HI was similar in patients with normal and abnormal oxygen pulse (P = .24) or peak oxygen consumption (P = .37).Fifty-nine percent of patients had cardiac and/or pulmonary limitation. A HI greater than 3.6 is associated with pulmonary dysfunction, but not cardiac dysfunction.