Two bronchial challenge protocols with breath-actuated dosimeters, Spira Elektro-2 and Mefar, with similar cumulative dose steps were compared in 28 patients with mild to moderate asthma. Methacholine challenges were performed after two different protocols at the same time of day in random order 3 or 4 days apart. The provocative dose of methacholine producing a 20% fall in forced expiratory volume in 1 second (PD20) was lower when determined by Spira than with the Mefar dosimeter (P < 0.05). Transition equations calculated by linear regression analysis were: PD20mefar = exp10[0.897 + 0.678(logPD20spira)] (P < 0.05; r = 0.62) and PD20spira = exp10[0.759 + 0.559 (log PD20mefar)] (P < 0.05; r = 0.62). The slopes were calculated by regressing the percentage fall in FEV1 on log10 (dose) and transformed as slope = 100/(regression coefficient + 10). The mean slope (95% CI) for Spira was 3.1 (2.6-3.7) and for Mefar 4.4 (3.6-5.1) (P < 0.005). Regression equations calculated by linear regression analysis were: slope(mefar) = 2.126 + 0.712 slope(spira) (P < 0.05; r = 0.51) and slope(spira) = 1.551 + 0.365 slope(mefar) (P < 0.05; r = 0.51). In conclusion, PD20 was smaller and the decline in FEV1/log(dose) curve steeper using the Spira compared with the Mefar protocol. The dose-response curves should be validated and transition equations calculated when bronchial reactivity to inhaled agents is compared, even while using apparently similar well-standardized dosimeter methods.