Standardized bronchial allergen provocation was performed twice in nineteen extrinsic, well defined, stable asthmatic patients, with an interval of median 15 days (range 14-19) to study the reproducibility of the bronchial response. Smoking and medications were withheld prior to the provocation after a rigid scheme. Ten-fold increasing concentrations of allergen solution 0.9 ml were inhaled by tidal volume breathing for 5 min with intervals of 10 min. The bronchial response to inhaled allergen was determined by forced expiratory volume in the first sec (FEV1) and by total resistance to breathing (Rt) determined by an opening interrupter method. The provocation was continued until an allergen concentration causing at least 20%, decrease of the postsaline FEV1 or a 40% increase in Rt was reached. A PC20-FEV1 and a PC40-Rt was calculated by interpolation on the log-dose-response curve. The reproducibility of PC20-FEV1 allergen was high with a 95% confidence interval (CI) for a single determination being the observed value +/- 0.83, ten-fold concentration difference, the intraclass correlation (IC) was 0.99 and the coefficient of variation 8.46%. Concerning PC40-Rt a 95% CI for a single determination was calculated being the observed value +/- 0.58, ten-fold concentration difference, IC was 0.99 and the coefficient of variation was 5.79%. No significant correlation was found between differences in pre-challenge FEV1 and Rt values and the corresponding PC20-FEV1 and PC40-Rt values. Least square regression between PC20-FEV1 and PC40-Rt was performed for the first and the second provocation (P less than 0.05). We conclude that bronchial allergen challenge performed in stable asthmatics is highly reproducible and as such a valuable test in the diagnosis of allergic asthma when connected with anamnesis, skin-prick test and the level of specific immunoglobulin E in peripheral blood.