Slopes of the dose-response curves to inhaled histamine or methacholine and PC20 were compared in 14 subjects with symptoms of bronchial hyperexcitability and in 10 normal subjects. Slopes were obtained by analyzing curves with 3 to 6 points by the least squares method. Only curves with satisfactory correlation coefficients corresponding to p values less than 0.05 were kept for analysis. Twelve of the 14 subjects with symptoms of bronchial hyperexcitability and 1 of the 10 normal subjects had slopes greater than 20 (percent change in FEV1/log histamine or methacholine concentration) (chi-square = 13.5, p less than 0.001). Eleven of the 14 subjects with symptoms of bronchial hyperexcitability and 1 of the 10 normal subjects had PC20 results less than or equal to 16 mg/ml (chi-square = 11.0, p less than 0.001). It is concluded that, using the method described in our study, slopes of the dose-response curve to inhaled histamine or methacholine are different in subjects with and without symptoms of bronchial hyperexcitability.