Inhaled long-acting β2-adrenergic agonists (LABAs) are recommended in the treatment of asthma as maintenance therapy when combined with an inhaled corticosteroid or when used alone to prevent exercise-induced bronchospasm.1 In addition, they are recommended alone or in combination with a long-acting inhaled anti-cholinergic or an inhaled corticosteroid as maintenance therapy for chronic obstructive pulmonary disease.2 Formoterol, salmeterol, and arformoterol are the only LABAs currently available in the United States, and they are marketed in various formulations alone or in combination with an inhaled corticosteroid. Although patents are still in force for several drug products containing a LABA (i.e., drug and device), the patents on the drug substance (i.e., the drug alone) have expired. Therefore, pharmaceutical firms have developed or are interested in developing alternative delivery devices for two of these drugs, and in some cases attempts are being made to develop a generic version. Obtaining U.S. Food and Drug Administration (FDA) approval of a generic LABA includes a requirement for an in vivo dose-response study to establish bioequivalence,3 and under varying circumstances, such a study is required by the European Medicines Agency4 and Canada’s Human Protection Branch.5 The purpose of such a study is to determine whether a test and reference product meet equivalence criteria based on a bioassay sensitive to differences in dose. If the study design is not sufficiently sensitive to detect a difference in two doses of the reference product, then it would not be possible to detect a difference between the test and reference products in the amount of drug delivered to the airways.6 In the United States, pharmacokinetic studies are not sufficient, by themselves, to establish a dose-response relationship,3 since plasma concentrations have not been proven to indicate where in the airways the drug is delivered and do not distinguish drug absorbed from the lungs and drug absorbed from the swallowed portion of the dose in the gastrointestinal tract.7 Thus, a pharmacodynamic end point is required to demonstrate the amount of drug delivered to the relevant receptors in the airways.8 Conventional bronchodilator studies, where the forced expiratory volume in 1 second (FEV1) is measured after a dose over time, have been used, but often the dose-response curve with this pharmacodynamic end point is relatively flat.9, 10 That is, during typical daytime studies in patients with stable asthma, one actuation may produce the same improvement in FEV1 as two actuations, since the response lies on the upper plateau portion of the dose-response curve.10 Dose-response studies of albuterol have demonstrated that bronchoprovocation with histamine or methacholine are the most sensitive methods of determining differences in doses when delivered by generic and brand chlorofluorocarbon (CFC) metered-dose inhalers (MDIs),11 between a CFC MDI and dry powder inhaler (DPI),12 and between CFC and hydrofluoroalkane (HFA) MDIs.13 Bronchoprovocation with methacholine has been used to compare the intensity and/or duration of formoterol delivered by a DPI or MDI.14–16 However, none of these studies has provided sufficient information to determine whether methacholine challenge can detect a 2-fold or smaller difference in dose and, thus, be useful as a bioassay to determine bioequivalence. Also, they do not provide information on the sample size required for a bioequivalence study. Since the Aerolizer (Foradil Aerolizer; Schering Corp., Kenilworth, NJ) is the only DPI formulation of formoterol available in the United States, it will serve as the reference product for subsequent studies of new delivery devices or generic formulations of this drug. Thus, the purpose of this study was to determine whether a methacholine-based clinical bioassay can be used to evaluate bioequivalence of formoterol (Aerolizer) and other formoterol-containing formulations. This would require that a highly significant dose-response relationship be present, which, in turn, requires a sufficiently steep dose-response slope relative to the response variability observed in the study. In addition, we propose possible improvements in study design that may further increase the statistical power of these bioequivalence methods.