Introduction : Cardiopulmonary exercise testing (CPET) is a method used to diagnose and stratify patients with known disease. The use of breath-by-breath analysis of exhaled air in a stress test helps us to understand the root cause and pathophysiology of pathological patterns causing clinical symptomatology. Aim : Using CPET to elucidate the role of chest deformity on human physical abilities, to determine the correlation of the measured parameters with the anthropometric index (AI) evaluating the severity of the deformity in patients with pectus excavatum (PE). Methods : The study included 30 paediatric patients with PE. According to AI, patients were divided into two groups, to patients with AI below 0.12 and patients with AI 0.12 and more. Patients underwent CPET using a breath-by-breath exhaled gas analysis method and continuous monitoring of cardiac parameters. Ventilation and cardiac parameters were statistically processed, the severity of the deformity was correlated with the results using the Pearson index. Results : The severity of the deformity according to AI had no effect on peak ventilation, VO2peak and WRpeak. By graphical representation and prognosis of the data, we demonstrated the relationship between the severity of the deformity and the efficiency of ventilation, OUES and O2Pulse at the peak of the exercise. Ventilation efficiency expressed as the slope of the VE/VCO2 curve also had a graphically dependent trend without statistical significance. Conclusion : CPET data obtained suggest that physical fitness parameters in patients with PE have a correlation with the severity of the deformity expressed by AI. The OUES parameter is a promising parameter for assessing the overall physical fitness of these patients and a parameter with potential use in deciding on the appropriateness of a therapeutic intervention. Key words : cardiopulmonary exercise testing, pectus excavatum, exercise tolerance