Asthmatic exacerbations result in part from constriction of airway smooth muscle. In this controlled trial, the use of bronchoscopically delivered thermoplasty to reduce the mass of airway smooth muscle resulted in fewer exacerbations among subjects with moderate or severe asthma. The incidence of adverse events was higher among subjects undergoing bronchial thermoplasty than among control subjects during the first 3 weeks after treatment. The use of bronchoscopically delivered thermoplasty to reduce the mass of airway smooth muscle resulted in fewer exacerbations among subjects with moderate or severe asthma. Many of the variable symptoms of asthma are thought to be due to the contraction of airway smooth muscle, leading to bronchoconstriction. 1 , 2 Increased airway smooth-muscle mass is a characteristic feature of asthma, particularly in persons with severe or fatal asthma. 3 , 4 Bronchial thermoplasty is a novel intervention in which controlled thermal energy is delivered to the airway wall during a series of bronchoscopies, resulting in a prolonged reduction of airway smooth-muscle mass. 5 In previous studies, we determined the amount and duration of energy to be delivered that result in modest thermal injury. 5 , 6 The treatment in humans of airways . . .