Brightling et al6 have described increased numbers of tryptase-positive mast cells infiltrating the airway smooth muscle in patients with mild and severe34 asthma in numbers that correlate with airway hyperresponsiveness.6,18 In the Severe Asthma Research Program Balzar et al16 reported that severe asthma was associated with an increase in numbers of bronchial mast cells staining positive for both tryptase and chymase and with BAL concentrations of PGD2, a lipid mediator associated with mast cells and shown to increase after allergen challenge.5 Here, CMA1 is linked to airway hyperresponsiveness.