OBJECTIVES: Oxidative stress is associated with the condition of cystic fibrosis (CF), but no guidelines exist for its assessment or treatment. Our aim was to evaluate a test that measures the blood antioxidant capability in CF children. METHODS: This antioxidant capability was assessed by the Kit Radicaux Libres (KRL) test in 44 CF children (24 boys). We recorded also anthropometric measures, pulmonary function, CF severity scores, and plasma nutritional and inflammatory parameters (proteins, vitamins, erythrocyte fatty acids, and micronutrients). We performed univariate and multivariate analyses with linear regression models. RESULTS: The mean age at the first KRL assessment was 12.2 ± 3.8 years. Factors that correlated with decreased antioxidant capacity were mostly related to the severity of pulmonary disease [ forced expiratory volume at 1 second (FEV1), acute exacerbation, and congestion. In multivariate analysis, the correlation between Brasfield score and erythrocyte antioxidant potential remained significant (β = - 0.611, p < 0.001). Among nutritional factors, the ω6/ω3 ratio was significantly correlated to erythrocyte antioxidant potential (β = - 1.213, p = 0.01). CONCLUSION: The blood antioxidant capability, measured by the KRL test, appears to be an interesting biomarker to evaluate oxidative stress in CF. This study suggests that the ω6/ω3 ratio should be regarded as a nutritional marker in antioxidant management in CF children.