The phospholipid fatty acid composition of erythrocytes from patients with cystic fibrosis with pancreatic insufficiency, patients with cystic fibrosis with normal pancreatic enzyme function, and normal subjects was determined. Increased percentages of palmitoleic acid and total eicosatrienoic acid and a decreased percentage of linoleic acid were observed only in the cystic fibrosis patients with pancreatic insufficiency.