The percentage fatty-acid composition of the various plasma-lipid fractions from cystic-fibrosis patients with and without pancreatic insufficiency, obligate heterozygotes, and normal subjects was determined. Only the cystic-fibrosis patients with pancreatic insufficiency had significantly abnormal fatty-acid composition. This general observation did not correlate with vitamin-E deficiency, age, sex, or severity of the disease. Thus the altered fatty-acid composition of the different plasma-lipid fractions appears to be a secondary consequence of the pancreatic insufficiency commonly associated with cystic fibrosis and not a direct metabolic defect related to the cystic-fibrosis gene itself.