This study was undertaken to evaluate the efficacy and safety of fluticasone propionate, an inhaled corticosteroid, in adolescents and adults with moderate asthma who were previously taking inhaled corticosteroids. After a 2-week, open-label screening period, a double-masked, randomized, parallel-group, dose-ranging study was conducted over 12 weeks in 21 outpatient centers throughout the United States. Patients (N = 304) > or = 12 years of age with moderate asthma previously treated with inhaled corticosteroids and beta-sympathomimetic bronchodilators were enrolled. Patients were assigned to receive placebo or fluticasone propionate 100, 250, or 500 micrograms twice daily via a metered-dose inhaler without a spacer device. These doses refer to the amount of fluticasone propionate released from the valve of the metered-dose inhaler; the corresponding doses released from the activator of the metered-dose inhaler are 88 micrograms, 220 micrograms, and 440 micrograms, respectively. Between baseline and end point, mean values of forced expiratory volume in 1 second decreased 0.31 L in the placebo group and improved 0.39 L, 0.30 L, and 0.43 L in patients receiving 100-micrograms, 250-micrograms, and 500-micrograms fluticasone propionate, respectively. The differences between placebo and all treatment groups were statistically significant. More patients were withdrawn from placebo (72%) than from fluticasone propionate (13% to 16%) because of failure to meet predetermined asthma stability criteria. Differences in baseline-to-end point changes in morning peak expiratory flow rate, physician overall assessments and patient-rated assessment of symptoms, and albuterol use for symptom control also significantly favored each fluticasone propionate group over placebo. There were essentially no differences in efficacy among the three fluticasone propionate groups. Treatment-related adverse events occurred in 8% of placebo-treated patients and 13% to 15% of fluticasone propionate-treated patients; these events were mainly localized to the oropharynx/ larynx. A 12-week course of fluticasone propionate (100, 250, and 500 micrograms twice daily) was well tolerated and more effective than placebo based on maintenance of asthma stability, pulmonary function tests, physician and patient assessments, and rescue bronchodilator use. No dose-related effects were observed with the dosages of fluticasone propionate used in this study.