||To determine the repeatability of bronchial responsiveness in awake young children, two methacholine challenge tests were performed on separate days in 16 children with stable asthma (mean age, 3 3/4 years). Methacholine was administered using a new method for quantitative delivery of aerosol that eliminates the effect of dilution of the aerosol by entrainment of air and enables quantitative delivery of aerosol according to body weight. Respiratory function was monitored by measurement of respiratory resistance by the interrupter technique (Rint), respiratory resistance and reactance at 5 Hz (Rrs5, Xrs5) by the impulse oscillation technique, transcutaneous measurements of oxygen (PtcO2), and specific airway resistance (sRaw). Repeatability was evaluated by determining the provocative dose that caused a defined percentage of change relative to baseline (PD%: Rint PD30, Rrs5 PD30, Xrs5 PD80, PtcO2 PD10, and sRaw PD80. Repeatability was estimated from the difference between the PD% obtained at the time of the two tests. Using the numeric value of these differences, the repeatability of Xrs5 PD60, PtcO2 PD10, and sRaw PD50 was [mean (SD)]: [0.8 (0.5)] [0.5 (0.4)] and [0.7 (0.6)] doubling doses, respectively. Rint PD30 and Rrs5 PD30 proved to be less reproducible: [1.2 (1)] and [1.6 (0.9)] doubling doses, respectively. The new method of aerosol delivery offers a means of standardizing the bronchoconstrictor stimulus, and the results show that estimates of bronchial responsiveness in young children can be obtained reproducibly within one doubling dose of methacholine.