Objective: To evaluate the oxygen uptake at peak exercise (VO2 peak) and to correlate with lung volume and capacity and body composition of children and adolescents with cystic fibrosis (CF). Methodology: Cross-sectional study with CF patients treated at the pediatric pulmonology outpatient clinic of a public hospital in Porto Alegre, aged [?] 7 years up to 18 years.Study assessments included: cardiopulmonary stress testing, plethysmography, and body composition assessment using body bioimpedance. Results: Thirty patients were studied, with a mean peak VO2 of 1.511 ± 0.539 liters in the population studied. Correlation analysis showed a strong correlation between peak VO2 in liters and lean mass (r = 0.77 and p <0.001), and strong and inverse correlation with fat percentage (r = -0.77 and p <0.001). ), a strong correlation with forced vital capacity (FVC) in liters (r = 0.72 and p <0.001) and forced expiratory volume in the first second (FEV1) also in liters (r = 0.69 and p <0.001). We observed a strong correlation between load (W) and lean mass and during CPET (r = 0.64 and p <0.001), and inversely with fat percentage (r = -0.64 and p <0.001). This study showed that patients with higher lean body composition have a better performance on cardiopulmonary testing, contributing to greater exercise tolerance.