There are no established guidelines for judging equivalence between inhaled medications. The principles of establishing bioequivalence on the basis of bioavailability and pharmacokinetics may not be applicable to inhaled medications with predominantly topical and minimal systemic effects. For inhaled β₂-agonists, the most practical method of showing in vivo therapeutic equivalence is by comparing relative potencies (RPs) of pharmacodynamic effects (bronchodilation and bronchoprotection). A range of doses that includes placebo should be studied in an appropriate design with adequate sample size, and relative potency should be estimated. Hydrofluoroalkane and chlorofluorocarbon salbutamol are bioequivalent for both their bronchodilator (RP, 1.08; 90% confidence interval, 0.95%, 1.23%) and bronchoprotective effects (RP, 1.08; 90% confidence interval, 0.81%, 1.46%) with similar safety profile. Eighteen subjects are required in a cross-over design to demonstrate bronchoprotective bioequivalence with a confidence interval of 67% to 150% for the relative potency (80% power). For salbutamol, this can be achieved with a comparison of 100 and 200 μg doses. Twelve subjects would suffice for a cumulative dose-response study for bronchodilator bioequivalence. For both outcomes, repeatability and quality control of measurements have to be ensured for an accurate interpretation of the results. (J Allergy Clin Immunol 1999;104:S243-5.)