Although some studies suggest that asthma deteriorates after reducing inhaled steroids, results of long-term studies indicate that this might not be true for all patients. The aim of this study was to determine the utility of the detection of a plateau on the concentration-response curves to inhaled methacholine as a marker for safely reducing the dose of inhaled budesonide in asthmatic patients who are well-controlled with a moderately high dose of this inhaled steroid. A total of 46 patients with moderate asthma, well-controlled for at least 6 months by treatment with 800 microg budesonide daily, were included in the study. Subjects were treated for a 2-week run-in period with their usual dose of budesonide. At the end of the run-in, all subjects were challenged with methacholine (0.095-200 mg x mL(-1)). Plateau responses, median effective concentration values, slopes and provocative concentration of methacholine causing a 20% fall in forced expiratory volume in one second (FEV1) values were measured. For the subsequent 12 weeks, patients were treated in an open design with budesonide at a reduced dose (200 microg once daily), and were asked to record their peak expiratory flow (PEF) in the morning and in the evening. In addition, asthma symptoms and use of rescue terbutaline were recorded in diaries. Plateaus were present in 24 patients, whereas 22 subjects showed concentration-response curves without evidence of a plateau. Ten patients in the nonplateau group deteriorated after reducing inhaled budesonide, compared to one patient in the plateau group (p = 0.002). In the nonplateau group, FEV1 decreased from a baseline value of 3.28+/-0.19 L to 2.94+/-0.20 L at week 12 (p<0.0001). Likewise, morning PEF decreased from 419+/-19 L x min(-1) at baseline to 394+/-19 L x min(-1) at week 12 (p = 0.02). By contrast, these variables remained unchanged in the plateau group. In conclusion, in asthmatic patients, well-controlled with a moderately high dose of budesonide, the detection of a plateau on the concentration-response curve to inhaled methacholine may be used as a marker for safely reducing the corticosteroid dose.