This study aims to systematically examine the cardiopulmonary functions in children with pectus excavatum and to compare the obtained findings with the Haller Index. Between September 2017 and June 2018, medical records of a total of 31 patients (27 males, 4 females; mean age: 14.8±2.0 years; range, 9 to 18 years) with pectus excavatum were retrospectively analyzed. The patients were divided into Group 1 (<2.5), Group 2 (2.5 to 3.19), and Group 3 (>3.2) according to the Haller Index. All groups were systematically evaluated based on pulmonary function tests and echocardiography. Forced vital capacity, forced expiratory volume in 1 second, and the forced expiratory volume in 1 second/ forced vital capacity ratio were calculated. Left ventricular enddiastolic diameter, ejection fraction, mitral valve prolapses, and right ventricular cavity in the apical four-chamber position were evaluated with echocardiography. Of the patients, 19.4% were in Group 1, 38.7% in Group 2, and 41.9% in Group 3. The mean Haller Index value was 3.09±0.64. According to pulmonary function test results, 16.1% of the patients had restrictive disease and 6.5% had obstructive disease. There was a negative correlation between the index and forced expiratory volume in 1 second and forced vital capacity, and there was a statistically significant decrease in these values, as the Haller Index increased (p<0.017). There was a significant difference in the ejection fraction among the groups (p<0.001) and, as the Haller Index increased, ejection fraction statistically significantly decreased. Our study results show a negative correlation between the severity of pectus excavatum and pulmonary dysfunction and, as the severity increases, left ventricular function may be affected by the deformity. As a result, there seems to be a significant relationship between the severity of the deformity and cardiopulmonary functions.