Background: The aim of the study was to assess the antioxidant status in cystic fibrosis (CF) patients compared to healthy controls. In order to determine the influence of nutrition on the level of the antioxidants, nutrient intake was also monitored in both groups at the time of the antioxidant assessment. Subjects and Methods: The authors measured the serum malondialdehyde levels in children with CF, n = 21; 9 females and 12 males, mean age: 8.71 years (6–12 years) and compared these values to the levels found in age-matched healthy control subjects, n = 24; 13 females and 11 males, mean age: 8.33 years (6–12 years). In order to assess the antioxidant status, catalase and superoxide dismutase activities in washed erythrocytes, glutathione peroxidase activity of heparinized whole blood and serum ascorbic acid, α-tocopherol and retinol levels were measured. Total antioxidant status (TAS) was also tested. The patients with CF received vitamin supplementation in doses prescribed in international guidelines (α-tocopherol: <10 years 100 mg daily, >10 years 200 mg daily, retinol: 2.5 mg daily, ascorbic acid: 100–200 mg daily). Results: Plasma levels of malondialdehyde were significantly higher (p < 0.05), superoxide dismutase activities were significantly lower (p < 0.05) in patients with cystic fibrosis. There were no significant differences in catalase, glutathione peroxidase activities and TAS levels between CF patients and control group. Plasma ascorbic acid, α-tocopherol and retinol levels were within normal limits in both groups. Conclusion: On the basis of the present results this regime failed to provide sufficient antioxidant protection. Therefore, the authors suggest that the daily dose of these antioxidants should be either increased or to administer in parenteral route to patients with severe form of the disease.