We studied the effects of supplementing patients with cystic fibrosis for 1 month with 3 g/day of a dietary supplement, containing 911 mg ω3 fatty acids, on the fatty acid composition of serum cholesterol esters, free fatty acids, phospholipids and triglycerides. At the end of the supplementation period, the content of total ω3 fatty acids was increased significantly in all lipid classes. The effect on individual ω3 fatty acids depended on the fatty acid and lipid class studied, increased with the dose fed and decreased with the weight of the patient. One month after the end of the supplementation period, fatty acid levels had returned to pretreatment values. These results demonstrate that enrichment of serum lipid classes with ω3 fatty acids is feasible in CF patients by dietary supplementation.