In cystic fibrosis (CF) lung disease, exhaled nitric oxide (FeNO) is not raised, but rather is normal or even decreased when measured at a single expiratory flow. FeNO measurements at several flow rates allow differentiation between alveolar and bronchial nitric oxide (NO) production. Extended FeNO measurements therefore should be useful to localize the FeNO deficit in CF airways. FeNO was measured in stable CF adults with moderate lung disease and in healthy controls. Bronchial NO fluxes (J(NO,Br)) and alveolar NO concentrations (C(Alv)) were calculated from FeNO measurements at flow rates of 100, 150 and 200 ml/s using a method previously described. Thirty-two adults were included in the study, 12 of whom had CF. CF adults had significantly lower FeNO values at all flow rates. The median J(NO,Br) was significantly lower in CF adults than in healthy controls [0.31 nl/s (range = 0.11-0.63) vs. 0.70 nl/s (0.27-3.52); P < 0.001], while the median C(Alv) was similar in both groups [1.7 ppb (0.3-3.9) vs. 1.2 (0.1-5.2)]. Pulmonary NO exchange did not differ significantly between subgroups of CF patients with and without chronic Pseudomonas aeruginosa infection. No significant correlation was detectable between FEV(1)/VC and J(NO,Br) and C(Alv), respectively. Extended FeNO measurements can separate alveolar and bronchial NO outputs in CF adults. The lower FeNO in adults with moderate to severe CF lung disease is likely to be the result of lower bronchial NO output.