Essential fatty acids (EFA) deficiency has been often reported in patients with cystic fibrosis (CF), particularly in those homozygous for the DF508 mutation. Clinical symptoms of CF may be influenced by correcting EFA deficiency. Nevertheless, the value of EFA supplementation in CF remains controversial. Within this multicentric and international randomized placebo-controlled trial it will be evaluated, according to recommendations of Cochrane analysis, beneficial effects of an oral supplementation with polyunsaturated fatty acids on selected biochemical and functional outcome parameters such as inflammatory biomarkers, incorporation into cell membrane phospholipids, lung function, exercise tolerance, clinical and nutritional status and properties of transepithelial ion transport. The study will be undertaken in a cohort of CF patients aged over 6 years old (60 patients), homozygous for the DF508 mutation and treated by Azithromycine. Supplementation will be performed with a triglyceride source at a daily dose of 60 mg/kg of omega-3 polyunsaturated fatty acids (Omega 3 Premiumâ, Laboratoires Ponroy, France). Before enrolled into the trial and during the study, patients will undergo nutritional assessment by evaluation of total and fat dietary intake and overall calorie intake using a 3-days diet records and a food frequency questionnaire. Plasma and erythrocyte membrane EFA profiles and inflammatory markers will be monitored in baseline conditions, at 3, at 6 and 12 months after starting the treatment. Lung function will be performed at each patient visit and an exercise test will be done before and at the end of the treatment. Properties of ion transport will be searched by sweat testing before and at the end of the treatment.