In a double-blind study of 2 parallel groups of 15 allergic asthmatic patients each, we investigated whether treatment with inhaled budesonide has a dose- and time-dependent effect on the degree of bronchial hyperreactivity. The patients were randomly allocated to treatment with either 200 or 800 micrograms budesonide per day for a period of 8 wk. The active treatment period was preceded by a selection period of 3 wk, and a single-blind placebo period of 2 wk. During these initial 5 wk the maintenance treatment of the patients, including cromolyn sodium and inhaled corticosteroids, was withheld. Spirometry and inhalation provocation tests with methacholine were carried out, and the symptom score was recorded every 2 wk. The methacholine provocation concentrations (geometric mean) causing a decrease in FEV1 of 20% (PC20) in the 200 and 800 micrograms/day treatment groups just before the active treatment period were 0.90 and 0.91 mg/ml, respectively. These values increased significantly to 1.21 and 1.84 mg/ml after 2 wk of treatment (p less than 0.05 and p less than 0.001, respectively) and to 1.55 and 2.74 mg/ml after 8 wk of treatment (p less than 0.01 and p less than 0.001). During the whole study period budesonide in a dosage of 800 micrograms/day induced a significantly larger change in PC20 than in a dosage of 200 micrograms/day. The FEV1 before treatment was 91 +/- 3% (SEM) and 84 +/- 2% of the predicted value in the 200 and 800 micrograms/day treatment groups, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)