Twenty young mild asthmatics (age range 7-22 yrs) were studied in order to determine the time course of bronchial hyperresponsiveness during inhaled steroid treatment and after steroid withdrawal. Study has been divided in three periods: Baseline (two weeks), Treatment (eight weeks) and placebo (eight weeks). Inhaled budesonide was administered during treatment period at a doses of 0.4 or 0.6 mg/ml twice a day, depending on age. After withdrawal, placebo was administered in the same way. Bronchial responsiveness (BR) to exercise was estimated trough the Bronchial Lability Index (BLI) defined as the amplitude of change of peak expiratory flow rate in a percentage related to the basal during a free running test. A BLI higher than 20% defined Bronchial Hyperresponsiveness (BHR). All patients presented BHR during the baseline period. Forced spirometry and exercise tests were performed every two weeks throughout the study. No changes in spirometric parameters (FVC, FEV1) were observed either during treatment or during placebo periods. A highly significant decrease in BR to exercise was observed during active drug treatment (BLI = 32.4 +/- 3.1 to 8.2 +/- 0.7 from basal to the end of the two months treatment period, mean +/- sd). During the placebo period 10/17 patients increased BLI to near baseline values, and 7/17 patients remained at normal levels. This behaviour was related to the severity of asthma during infancy. Our results confirm the close association between inflammation and BHR.(ABSTRACT TRUNCATED AT 250 WORDS)