A subset of patients with asthma is known to have progressive loss of lung function despite treatment with corticosteroids. The aim of the present study was to identify risk factors of decline in forced expiratory volume in one second (FEV(1)) in patients with difficult-to-treat asthma. In total, 136 nonsmoking patients with difficult-to-treat asthma were recruited between 1998 and 1999. Follow-up assessment was performed 5-6 yrs later in 98 patients. The predictive effect of clinical characteristics and inflammatory markers were analysed at baseline (asthma onset and duration, atopy, airway hyperresponsiveness, blood and sputum eosinophils, and the fraction of nitric oxide in exhaled air (F(eNO))) on subsequent decline in post-bronchodilator FEV(1). Patients with high F(eNO) (> or =20 ppb) had an excess decline of 40.3 (95% confidence interval (CI) 7.3-73.2) mL.yr(-1) compared to patients with low F(eNO). F(eNO) > or =20 ppb was associated with a relative risk of 1.9 (95% CI, 1.1-2.6) of having an accelerated (> or =25 mL.yr(-1)) decline in FEV(1). In patients with baseline FEV(1) > or =80% of predicted, this relationship was even stronger: 90 versus 29% had accelerated decline in FEV(1) (F(eNO) > or =20 ppb versus F(eNO) <20 ppb respectively; relative risk 3.1 (95% CI, 1.7-3.4). Exhaled nitric oxide is a predictor of accelerated decline in lung function in patients with difficult-to-treat asthma, particularly if forced expiratory volume in one second is still normal.