The relative importance of the usual diet in serum phospholipids in subjects with cystic fibrosis (CF) has been poorly studied. To compare the fatty acid profile in serum phospholipids from adult CF subjects with that of healthy subjects, and determine the role of the normal diet in this profile, we studied thirty-seven adult CF subjects with stable pulmonary disease and thirty-seven healthy controls matched for age, sex and nutritional status. A dietary questionnaire was obtained, anthropometric data were recorded, and the fatty acid profile measured by GLC. Compared with the controls, the percentages of myristic, palmitoleic and stearic acids and total MUFA were significantly higher in the CF group, and DHA, linoleic acid, total PUFA and n−6 fatty acids were significantly lower in the CF group. The CF subjects with worse pulmonary function and with pancreatic insufficiency had significantly lower levels of linoleic and n−6 fatty acids. The total energy intake was significantly higher in the CF subjects, although the energy distribution in the CF subjects and the controls was not different for the carbohydrates, lipids and proteins. No differences were detected in fat intake for MUFA (51 (sd 4) v. 52 (sd 4) %) or saturated fatty acids (33·5 (sd 5) v. 31·2 (sd 3·8) %), but the PUFA were slightly lower in the CF subjects (15·4 (sd 4·5) v. 17·4 (sd 4·2) %; P=0·02). The usual dietary intake of fatty acids by adult CF subjects does not appear to explain the difference in the fatty acid profile compared with controls. This suggests an abnormal fatty acid metabolism in CF subjects.