Fluticasone propionate (FP) administered via metered-dose inhaler is a potent corticosteroid effective in the treatment of asthma. To evaluate the efficacy and safety of FP powder administered via a breath-activated inhaler (Diskhaler), a multicenter, double-blind, randomized, placebo-controlled, parallel-group study was conducted in adolescent and adult patients (n = 331) with mild-to-moderate asthma previously treated with beta 2-agonist therapy alone. Patients received FP powder 50, 100, or 250 micrograms or placebo twice daily for 12 weeks. FP-treated patients compared with placebo-treated patients had significantly (p < 0.001) greater improvements in morning predose forced expiratory volume in 1 sec (21-22% increase vs. 9%). Improvement in morning peak flow rate were also significantly (p < 0.001) greater with FP than with placebo (8-10% increase vs. 2% increase). There was also a significant overall treatment difference in the frequency of inhaled albuterol use (p < 0.001) and number of nighttime awakenings due to asthma (p = 0.005). There were no statistically significant difference among the FP treatment groups in any outcome measure. Physicians' global assessments also indicated significant (p < 0.001) differences in efficacy, with 67-74% of FP-treated patients rated as having "effective" or "very effective" treatment compared with 41% of placebo-treated patients. Significant beneficial effects of FP were observed in lung function and diary card parameters after just 1 week of treatment. Adverse events were similar across treatment groups and primarily related to local irritation. Effect on hypothalamic-pituitary-adrenal axis function was minimal. In summary, all three dosages of inhaled FP powder were well tolerated and improved various asthma-related variables. Improvements in pulmonary function, beyond those achieved with beta 2-agonist therapy alone, were maintained for the duration of the 12-week study.