Nocturnal asthma symptoms and impaired lung function at night are related to inflammatory activity in the peripheral lung compartment. Exhaled nitric oxide (NO) measurement at multiple exhalation flow rates can be used to separately assess alveolar and bronchial NO production and inflammation. The authors hypothesised that asthmatic patients with nocturnal symptoms have a higher alveolar NO concentration than those with only daytime symptoms. The authors asked 40 patients with newly-diagnosed steroid-naïve asthma about their nocturnal asthma symptoms through the use of a written questionnaire. Alveolar NO concentration and bronchial NO flux were assessed in the 40 asthmatics and 40 healthy controls. Nineteen of the 40 patients reported nocturnal symptoms. Patients with nocturnal symptoms had a higher alveolar NO concentration (1.7+/-0.3 (mean+/-SEM) parts per billion (ppb)) than patients without nocturnal symptoms (0.8+/-0.3 ppb, p=0.012) or healthy controls (1.0+/-0.1 ppb, p=0.032). Bronchial NO flux was higher both in patients with (2.4+/-0.4 nL x s(-1), p<0.001) and without (2.6+/-0.4 nL x s(-1), p<0.001) nocturnal symptoms, compared to controls (0.7+/-0.1 nL x s(-1)). Nocturnal symptoms in asthmatic patients are related to a higher alveolar nitric oxide concentration. The results suggest that assessment of alveolar nitric oxide concentration can be used to detect the parenchymal inflammation in asthmatic patients with nocturnal symptoms.