The aim of the present study was to see whether measurements of airway hyperresponsiveness (AHR) and nitric oxide (NO) in exhaled air (ENO) either separately or in combination, could differentiate between asthmatics and healthy control subjects in a population based survey.In central Norway 8,571 adolescents participated in a large-scale epidemiological survey (Young Helseundersùkelsen i Nord-Trùndelag (Health Survey in NorthTrùndelag; HUNT). Asthmatic symptoms when exposed to pollen, pets or house-dust were reported by 7.8% (suspected asthmatics), while 56% reported no asthmatic or allergic symptoms (control subjects). From these respective groups 151 and 213 adolescents were investigated with allergy screening, measurements of exhaled and nasal NO, and methacholine challenge test.AHR (provocative dose of methacholine causing a 20% fall in forced expiratory volume in one second (PD20) <2 mg) was confirmed in 75% of the suspected asthmatics versus 25% of the control subjects, whereas 52% versus 20% had elevated levels of ENO ($8 parts per billion (ppb)). ENO and dose response ratio to methacholine (DRR) were positively correlated (r=0.41, p<0.001). ENO was significantly elevated in atopic versus nonatopic suspected asthmatics (11.7 ppb and 5.6 ppb respectively, p<0.001). Suspected asthmatics with both AHR and atopy had the highest levels of ENO (14.2 ppb).It is concluded that measurements of nitric oxide in exhaled air alone are not a useful tool in diagnosing asthma in population surveys, but that the combination of airway hyperresponsiveness and elevated nitric oxide in exhaled air is a very specific finding for allergic asthma. The use of dose response ratio to methacholine did not provide any additional information to the provocative dose of methacholine causing a 20% fall in forced expiratory volume in one second in this study. Eur Respir J 2000; 15: 849±855.