Lung function and determination of the bronchial responsiveness to methacholine were performed in two groups of Swiss army recruits aged 20 years (total 233), where 144 had respiratory complaints or a history of prior asthma and 89 were asymptomatic, among whom 26 had recently suffered from upper respiratory tract infection (URTI). A bronchial obstruction (ratio FEV1/FVC less than 73%) was observed among 6% of the subjects with symptoms, 6% of the asymptomatic control subjects and 4% in the group with recent URTI. The differences are not significant. A bronchial hyperresponsiveness (defined as PD20 less than 900 mcg) was observed in 45% of the symptomatic subjects (54% in 109 recruits complaining of dyspnea on exertion) against 3% in the control group (p less than 0.001) and 12% in the group with recent URTI (NS, p = 0.14). Among subjects complaining of dyspnea on exertion, more than half have an obvious bronchial hyperresponsiveness at a level usually observed in asthmatics, most of them without any clinical finding or permanent bronchial obstruction. Although the sensitivity of the determination of bronchial responsiveness is low (47%) its specificity is high (97%). The test allows detection of some subjects with asthma among young subjects complaining of respiratory troubles in spite of normal clinical examination and lung function. A normal level of bronchial responsiveness practically excludes asthma.