Background: Long-acting β₂-sympathomimetic agonists such as salmeterol have been proved safe and effective for the treatment of asthma. However, controversy still exists as to the appropriateness of scheduled long-term therapy with these agents. Objective: This study assessed the degree of bronchodilation provided by treatment with salmeterol for a period of 52 weeks and evaluated bronchial hyperresponsiveness to methacholine during and after the treatment period. Methods: Three hundred fifty-two patients with mild to moderate asthma were assessed by 12-hour serial spirometry and serial methacholine challenge tests. Results: The mean area under the FEV₁ curve above baseline over 12 hours after drug at day 1 was significantly greater with salmeterol powder compared with placebo (5.06 liter hours vs 0.77 L/h) and did not change significantly over 1 year. The mean increase in the log₂ of the provocative cumulative methacholine dose producing a 20% decrease in FEV₁ (PD₂₀FEV₁) during treatment was significantly higher in the salmeterol-treated patients than in the placebo group (1.02 doubling doses vs 0.43 doubling doses at week 4, 1.06 doubling doses vs 0.41 doubling doses at week 24). At week 52 the increase from baseline in log₂PD₂₀FEV₁ was not significantly different between salmeterol and placebo (1.08 vs 0.69 doubling doses). Seven days after treatment the log₂PD₂₀FEV₁ was –0.60 doubling doses lower than baseline for salmeterol compared with 0.10 doubling doses for placebo (P = .031). Long-term salmeterol use was not associated with a deleterious effect on asthma control during and after treatment. Conclusion: This study demonstrates that the bronchodilator properties of salmeterol are sustained over 52 weeks and that bronchial hyperresponsiveness to methacholine is decreased to a modest degree during treatment. Clinically significant increases in hyperresponsiveness did not develop after discontinuation of salmeterol treatment. (J Allergy Clin Immunol 1999;104:1189-97.)