The main objective of this study was to examine the perception of the first symptom during methacholine bronchoconstriction as soon as it occurred, and in the second instance to quantify the intensity of the breathlessness by means of the Borg Scale performed at the end of challenge so to not determine any difficulties for identification of the first symptom. A methacholine challenge test was carried out in 139 symptom-free asthmatics with a normal pulmonary function. When the first symptom was reported by the subject, the forced expiratory volume in one second (FEV1) was measured immediately. FEV1 was expressed as a percentage of the best personal value (FEV1%BPV). The intensity of the breathlessness was rated using a modified Borg Scale at the end of the challenge test so as not to confuse the patient. Seven subjects felt nothing during challenge. The first symptoms varied: constriction behind the breastbone (49%), inspiratory shortness (16.5%), coughing (10%), wheezing, throat constriction, general chest tightness, pain behind the sternum, and a sensation of rheum behind the sternum. FEV1%BPV at the first symptom was 80.5 +/- 10 (range 41-99). No symptom was perceived by 42% of the subjects within the 80-100 range of FEV1%BPV. FEV1%BPV at the first symptom was related to the log of the provocative dose causing a 20% fall in FEV1 (r = 0.2, p < 0.05). An inverse correlation between Borg Score and final FEV1%BPV (r = -0.25, p < 0.01) was found at the end of challenge. A subgroup of 39 subjects with similar final FEV1%BPV values (68-72) showed a correlation between the Borg Score at end of challenge and FEV1%BPV at the first symptom (r = 0.59, p < 0.001). The first symptom of the methacholine-induced bronchoconstriction varies in asthmatics and may be atypical, the bronchoconstriction level at which it is felt also varies among individuals, highly methacholine-responsive subjects perceive the bronchoconstriction later, while late perceivers of the first symptom show less intense breathlessness at the end of challenge. The measurement of the patient's ability to perceive asthmatic symptoms during the methacholine challenge test could be used to single out poor perceivers.