Isocapnic hyperventilation with cold air (IHCA) is a reliable technique for assessing indirect bronchial hyperresponsiveness in patients with asthma. Impedance measurement of the respiratory system by the forced pseudorandom noise oscillation technique is a sensitive technique to assess changes in bronchial tone after IHCA. The aim of this study was to evaluate the effect of 6 weeks of treatment with beclomethasone dipropionate, 1,000 microg x day-1, on IHCA in asthmatic patients, measured with both forced oscillation technique and flow-volume recordings. Forty patients with mild asthma were included in this double-blind, placebo-controlled parallel-group study. Stratification on the basis of sex was performed to overcome differences in airway diameter. At entry and every 2 weeks during the treatment period, IHCA was performed and patient diaries were evaluated. Characteristic changes in forced oscillation parameters after IHCA were observed in all patients. After 6 weeks of treatment, BDP-treated patients showed statistically significant differences in impedance measurements after IHCA, manifested by significant attenuation of resistance at 8 Hz (p<0.01), slope of the frequency-resistance curve (p<0.01), reactance at 8 Hz (p=0.01), and resonant frequency (f0) (p<0.02). Flow-volume recordings showed only a statistically significant change in the decrease of inspiratory vital capacity (IVC) (p=0.01). Furthermore, a significant correlation was observed between serum immunoglobulin E (IgE) levels and the effect of BDP on IHCA, measured with forced oscillation technique. In this study, beclomethasone dipropionate, 1,000 microg x day(-1) for 6 weeks, decreased indirect bronchial hyperresponsiveness as assessed by cold air bronchoprovocation in asthmatic patients. The forced oscillation technique proved a more sensitive method of detecting changes in bronchial tone than flow-volume recordings.