In asthmatics, the score of bronchoconstriction-associated breathlessness at 20% fall in forced expiratory volume at first second (FEV1) evaluated on a Borg scale (PS20) is a tool successfully used to measure the perception of symptoms. This prospective laboratory study evaluated the applicability of PS20 to assess the breathlessness induced by ultrasonically nebulized distilled water (UNDW) and methacholine (M) and its reproducibility. Twenty-two mild and clinically stable asthmatic patients performed UNDW and M challenge tests. The PS20 was calculated by linear interpolation of the last two points of the perception/fall in FEV1 curve of the UNDW and M tests. The reproducibility of PS20 M was assessed by repeating measurements on 2 separate days by 3 weeks. PS20 UNDW and PS20 M did not differ and were respectively 1.82 +/- 1.85 and 2.03 +/- 1.86. They were significantly related (rs=0.63; p<0.01) and the bias between PS20 UNDW and PS20 M was -0.21 with the limits of agreement ranging from -3.2 to 3.6. The intraclass correlation coefficient for repeated measurement of PS20 M was 0.82; the bias between the two measurements was 0.2 with the limits of agreement ranging from -2.8 to 3.2. All patients had a measurable breathlessness perception degree on a Borg scale during both distilled water challenges and methacholine. Asthmatic patients with normal, exaggerated or poor breathlessness perception were also similar for both stimuli. In addition, PS20 showed a good reproducibility and this allows the serial evaluation of patient's breathlessness perception by this technique in clinical settings and in the physiology laboratory.