Methacholine challenges are widely used to measure airway hyperresponsiveness (AHR) as a diagnostic aid in the assessment of individuals with asthma-like symptoms and normal resting expiratory flow rates. Many of us believe that the major clinical value of the methacholine challenge rests in its very high diagnostic sensitivity and negative predictive value,1 indicating that a negative methacholine challenge result should exclude current asthma with reasonable certainty. Several years ago, the American Thoracic Society (ATS) published detailed methods for two of the more common methacholine inhalation procedures.2 With exception of the pattern of inhalation, all other aspects of the methods are identical, including methacholine concentrations, timing between inhalations, number and timing of FEV1 maneuvers, and calculation of the provocation concentration producing a 20% fall in FEV1 (PC20). The first is the 2-min tidal breathing method, in which methacholine is inhaled for 2 min of tidal breathing of an aerosol generated continuously from a jet nebulizer calibrated to an output of 0.13 mL/min, exposing the individual to approximately 90 μL of aerosol at each concentration. The second requires five slow, deep inhalations to total lung capacity (TLC) with a 5-s breathhold from a breath-activated dosimeter calibrated at 9 μL per breath delivering 45 μL of aerosol at each concentration. While the tidal breathing method exposes the individual to twice the amount of aerosol at each concentration, the two methods were believed to give the same result.2 This was based on a study3 in 10 individuals using inhaled histamine, and was supported by a more recent study4 in 11 subjects receiving methacholine.