The aim of this study was to assess responsiveness to inhaled histamine and methacholine using PC20, the concentration causing a 20% fall in forced expiratory volume in one second (FEV1) in 28 subjects with chronic bronchitis excluding asthma. Eighteen subjects had airway hyperexcitability, as demonstrated by a PC20 less than 16 mg/ml. The response to histamine and methacholine was not significantly different and the correlation coefficient was 0.75. Baseline airway caliber was related to the level of hyperexcitability, as 14 of 14 subjects with a FEV1 to forced vital capacity ratio less than 95% pred and/or a forced expiratory flow during the middle half of the FVC less than 80% pred, and only 4 of 10 subjects with normal spirometry, had a PC20 less than 16 mg/ml (x2 = 15.6, p less than 0.001). There was no correlation between blood or sputum eosinophilia, total IgE levels, or atopy and the level of airway hyperexcitability. Spontaneous daily variations of peak expiratory flow rates were significantly (t = 2.43, p less than 0.05) more pronounced in subjects with PC20 less than 16 mg/ml. We conclude that airway hyperexcitability assessed with PC20 is often present in chronic bronchitis and more often so in the presence of airway obstruction.