OBJECTIVE A dose-ranging study was conducted to evaluate the efficacy and safety of a new long-acting, selective beta 2-adrenoceptor agonist, salmeterol. METHODS Adolescents and adults (N = 160) with mild-to-moderate asthma received salmeterol (10.5, 21, 42, or 84 micrograms) or placebo by metered-dose inhaler twice daily for 1 week. Twelve-hour serial spirometry measurements were performed on the first and last days of treatment, and patients recorded their peak expiratory flow (PEF) twice daily on diary cards. RESULTS On day 1, salmeterol produced greater bronchodilation than placebo (p = 0.001), and both the 42-micrograms and 84-micrograms doses of salmeterol were significantly more effective in improving FEV1 responses than the two lower doses of salmeterol (p < 0.05). After 1 week of treatment, all but the 21-micrograms dose of salmeterol remained statistically superior to placebo (p < 0.01), but significant differences between salmeterol doses were no longer evident, despite an apparent dose-response effect. Only the 42-micrograms and 84-micrograms doses of salmeterol sustained bronchodilation for 12 h in the majority of patients at both treatment days. The degree of improvement in morning and evening PEF was also found to be dose related. There was no significant difference among treatment groups in the overall incidence of adverse events; however, pharmacologically predictable events (eg, tremor) occurred significantly more often with salmeterol, 84 micrograms. CONCLUSIONS Salmeterol, 42 micrograms, was similar in efficacy to 84 micrograms but was associated with a lower incidence of adverse events. Salmeterol, 42 micrograms twice daily, is a safe and effective dosage for patients with mild-to-moderate asthma who are persistently symptomatic and require maintenance bronchodilator therapy.