In this 4-week, multicenter, double-blind, randomized, parallel group study, the dose-effect relationship of four doses of inhaled fluticasone propionate (50, 100, 200, and 400 micrograms twice daily) was investigated and compared with beclomethasone dipropionate, 200 micrograms twice daily. A total of 672 patients with moderate asthma currently receiving 1,000 micrograms/d or less of an inhaled steroid were recruited. The study demonstrated a significant dose-related improvement in lung function with fluticasone propionate. Linear dose-related increases were observed in morning (increase per doubling dose was 4.3 L/min; 95 percent confidence interval [CI], 1.8, 6.8 L/min; p = 0.001) and evening peak expiratory flow rate (PEFR) (increase per doubling dose was 3.0 L/min; 95 percent CI, 0.5, 5.5 L/min; p = 0.017), clinic lung function (at 4 weeks, increase in percent predicted PEFR per doubling dose = 1.1 percent; 95 percent CI, 0.2, 2.1 percent; p = 0.022; increase in percent predicted FEV1 per doubling dose = 1.1 percent; 95 percent CI, 0.3, 1.9 percent; p = 0.10:increase in percent predicted FVC per doubling dose = 1.3 percent, 95 percent CI, 0.5, 2.1 percent; p = 0.001), and the percentage of symptom-free days over days 1 to 14 of treatment (increase per doubling dose = 1.9, 95 percent CI, 0.0, 3.9; p = 0.048). There was also a dose-related reduction in extra bronchodilator usage (days 1 to 14 p = 0.002; days 15 to 28 p = 0.01). In addition, there was a significant decrease in diurnal variation with increasing doses of fluticasone propionate (decrease per doubling dose = 2.0 L/min, 95 percent CI, 0.4; p = 0.024). The number of asthma exacerbations was also reduced as the dose of fluticasone propionate increased. Fluticasone propionate was well tolerated, adverse events were few, and there was a similar incidence in all groups. Furthermore, there was no evidence of any hypothalamic pituitary adrenal axis suppression. The data from the study were consistent with other clinical studies that have shown fluticasone propionate to be more potent than beclomethasone dipropionate in terms of improvement in lung function. In conclusion, this study provided evidence of a dose-related improvement in asthma control for fluticasone propionate in the dose range 100 to 800 micrograms daily, in patients with moderate asthma.