We investigated the use of soluble transferrin receptor (sTfR), together with more traditional measurements such as iron, transferrin and ferritin, to assess iron status in 70 adult cystic fibrosis patients. sTfR concentration was measured using particle-enhanced immunonephelometry. We found that 69% of subjects as determined by transferrin saturation had iron deficiency, but only 29% as determined by sTfR and 11% as determined by ferritin. We found a significant correlation between C-reactive protein (CRP) and both ferritin (r = 0.38, P = 0.0001) and transferrin saturation (r = -0.54, P < 0.0001). In addition, because the CRP concentration was elevated in 64% of subjects, we feel that the transferrin saturation was overestimating, and the ferritin underestimating, iron deficiency in these patients. The sTfR concentration, on the other hand, is unaffected by the acute-phase response and was therefore thought to be the most useful test for detecting iron deficiency in this group of patients.