Patients with chronic hypoxaemia develop secondary polycythaemia that improves oxygen-carrying capacity. Therefore, normal haemoglobin and haematocrit values in the presence of chronic arterial hypoxaemia in cystic fibrosis constitute 'relative anaemia'. We sought to determine the cause of this relative anaemia in patients with cystic fibrosis. We studied haematological indices and oxygen saturation in healthy volunteers (n=17) and in adult patients with cystic fibrosis (n=15). Patients with cystic fibrosis had lower resting arterial oxygen saturation when compared with normal volunteers (P<0.0001), and exercise led to a greater reduction in arterial oxygen saturation (P<0.0001). However, haemoglobin and haematocrit values in patients with cystic fibrosis did not significantly differ from normal volunteers. Serum iron (P=0.002), transferrin (P=0.02), and total iron-binding capacity (P=0.01) were lower in patients with cystic fibrosis. There were no significant differences in serum ferritin, percentage iron saturation, serum erythropoietin or red cell volume between the groups. The data presented demonstrate a characteristic picture of anaemia of chronic disease in adult patients with cystic fibrosis, except for normal haemoglobin and haematocrit values. Normal haemoglobin and haematocrit values in patients with cystic fibrosis appear to represent a combination of the effects of arterial hypoxaemia promoting polycythaemia, counterbalanced by chronic inflammation promoting anaemia of chronic disease.