Dose-response relationships with inhaled corticosteroids in the treatment of asthma have been difficult to establish. A multicenter, double-blind, parallel-group study was conducted to evaluate the clinical efficacy and safety of low doses of inhaled fluticasone propionate (FP) in patients with mild to moderate asthma. Methacholine challenge testing was conducted in addition to measurement of traditional efficacy variables. After a single-blind screening period, 138 patients > or = 12 years of age were randomly assigned to receive placebo, FP 50 microg, or FP 100 microg, twice daily for 8 weeks. The results of methacholine challenge testing averaged over all visits favored FP 200 microg/day over placebo and FP 100 microg/day (p < 0.05); there were no significant differences between placebo and FP 100 microg/day. Mean changes from baseline to endpoint favored each dose of FP over placebo based on forced expiratory volume in 1 sec (FEV1), patient-measured peak expiratory flow (PEF), total symptom scores, and rescue bronchodilator use (p < 0.05); there were no differences in these parameters between the two doses of FP. The addition of methacholine challenge testing allowed definition of a dose-response relationship that was not apparent with traditional efficacy variables.