Functional iron deficiency (transferrin saturation < 16%) was found in 44 (62%) of 71 adult cystic fibrosis (CF) patients. Haemoglobin concentration and mean cell volume were lower in iron-deficient patients, in whom there was a non-significant trend for lower serum ferritin. Ten iron-deficient patients and two patients with transferrin saturation > = 16% (normal iron) were anaemic. There were no significant differences between iron-deficient and normal-iron patients in intake of calories, protein, iron and vitamin C as determined by 4-day records of dietary intake. Dietary iron deficiency is not an important factor in functional iron deficiency in adult CF patients. Impairment of absorption by exogenous pancreatic enzyme supplements is unlikely to be significant as enzyme intake was the same in the two groups. Iron-deficient patients had lower Shwachman-Kulczycki scores and lower percent predicted forced expiratory volume in 1 s (FEV1% predicted) and forced vital capacity (FVC% predicted). There was a non-significant trend for higher values of white cell count and plasma viscosity in the iron-deficient group. Chronic inflammation is likely to be the primary cause of functional iron deficiency in adult CF patients. Fifteen patients completed 3-month courses of oral iron replacement with no deterioration in pulmonary function, but with no effect on haemoglobin concentration.