The authors discuss the efficacy of methacholine challenge to discriminate fit subjects to military service. We evaluated the relation between bronchial hyperreactivity and clinical symptoms, airways caliber and atopic status in a group of italian conscripts who reported to have bronchial asthma. Five-hundred-four subjects were studied. Bronchial hyperreactivity was measured by methacholine test, and atopic status was assessed by skin-tests. A measurable PC20 FEV1 was detected in 424 subjects. On the basis of the methacholine threshold concentration the overall sample was divided in four categories. The four categories differed as regards onset of disease, lung function and skin reactivity towards Dermatophagoides Pter, whereas no difference was found as regards skin reactivity towards Grass. In the group evaluated in spring, the four categories differed as regards skin reactivity towards Grass. In conclusion we found that bronchial hyperreactivity is related to clinical history, lung function and atopic status; the measurement of bronchial hyperreactivity is important to evaluate conscripts referring bronchial asthma.