Exhaled nitric oxide (NO) is thought to be a marker of asthmatic inflammation. Levels in cystic fibrosis (CF) are generally low. This study aimed to measure exhaled NO in CF patients at high risk of developing ABPA and patients at low risk. We studied nine patients at high risk of developing ABPA and 36 at low risk. The two groups were similar in age and spirometry. All patients in the high-risk group were taking oral or inhaled glucocorticoids, compared to 56% in the low-risk group (P=0.02). The exhaled NO levels were lower in the high-risk group than in the low-risk group (2.0 vs. 3.6 ppb), mean difference (95% CI) 1.6 (-3.6 to 0.4) ppb, P=0.001. On subgroup analysis of patients on oral glucocorticoids, the exhaled NO levels were significantly lower in patients with a high risk of developing ABPA (n=7) than patients with a low risk (n=8) (P=0.011). The number of patients who were on inhaled, but not oral glucocorticoids was too small to analyse usefully. Exhaled NO levels were lower in CF patients with a high risk of developing ABPA and on glucocorticoids. This may be because oral glucocorticoids exert a greater effect on exhaled NO than inhaled glucocorticoids. Alternatively, inducible nitric oxide synthase may be down-regulated by Aspergillus toxin.