The concentration of nitric oxide (NO) is increased in the exhaled air of asthmatic patients and may reflect cytokine-mediated inflammation in the airways. We investigated whether allergen-induced inflammation causes an elevation in the level of exhaled NO. Of 25 patients who underwent allergen challenge, 16 developed dual early and late responses, whereas eight had a single early response. In the patients with a dual response, the maximal fall in FEV1 during the late response was 26.8 +/- 4.2% at 9 h and there was a significant increase in the level of exhaled NO (maximal increase of 59.4 +/- 9.8%) 10 h after challenge. There was a significant relationship between the size of the late response and the increase in exhaled NO (r = 0.75, p < 0.01). In patients who have a single early response, there was no significant increase in exhaled NO, with the exception of a single time point at 21 h. In five patients given a control challenge with methacholine there was no change in exhaled NO. There was no increase in exhaled NO after inhaled histamine in any of the patient groups. We conclude that the late asthmatic response to allergen is associated with elevated exhaled NO concentrations and that this provides further evidence that exhaled NO may reflect allergic inflammation in asthmatic airways, and may be a useful marker in monitoring asthma and its response to anti-inflammatory treatments. Whether endogenously produced NO plays a pathophysiologic role in the late response remains to be determined.