The frequency with which disagreement occurs between the clinician's diagnosis of asthma and the objective measurement of methacholine airway responsiveness is not known. We have examined this in 51 consecutive new adult patients presenting with symptoms consistent with asthma but with normal spirometry. Disagreement between the physician's assessment and the result of the methacholine test occurred in 20 patients (39%). Of these, 13 had a clinical diagnosis of asthma but normal responsiveness and 7 had an unexpected increase in methacholine responsiveness. There was no difference in the type of symptoms experienced whether or not there was agreement between the test and the physician's assessment. Thus, an objective measurement like a methacholine inhalation test is clinically useful when spirometry is normal. A normal methacholine test in the presence of symptoms suggestive of asthma should alert the physician to the need for further investigation.