Rationale: Bronchial thermoplasty (BT) is designed to reduce airway smooth muscle and improve asthma control.Objectives: This study was conducted to determine the safety and efficacy of this procedure in subjects with symptomatic, severe asthma.Methods: Adults who were symptomatic despite treatment with fluticasone or equivalent at more than 750 μg/day, a long-acting β2-agonist, and other medications, which could include 30 mg or less of oral prednisolone/day, were randomized to BT or to a control group. After treatment, subjects entered a 16-week steroid stable phase (Weeks 6–22), a 14-week steroid wean phase (Weeks 22–36), and a 16-week reduced steroid phase (Weeks 36–52).Measurements and Main Results: BT resulted in a transient worsening of asthma symptoms. Seven hospitalizations for respiratory symptoms occurred in 4 of 15 BT subjects during the treatment period. Five hospitalizations were within 3 days of treatment. Two subjects had segmental collapse involving the most recently treated lobe; one required bronchoscopy and aspiration of a mucus plug. There were no hospitalizations during this period in the 17 control subjects. The rate of hospitalizations was similar in both groups in the post-treatment period. At 22 weeks, BT subjects had significant improvements versus control subjects in rescue medication use (−26.6 ± 40.1 vs. −1.5 ± 11.7 puffs/7 d, P < 0.05), prebronchodilator FEV1% predicted (14.9 ± 17.4 vs. −0.94 ± 22.3%, P = 0.04), and Asthma Control Questionnaire scores (−1.04 ± 1.03 vs. −0.13 ± 1.00, P = 0.02). Improvements in rescue medication use and Asthma Control Questionnaire scores remained significantly different from those of controls at 52 weeks.Conclusions: BT is associated with a short-term increase in asthma-related morbidity. However, there is preliminary evidence of long-lasting improvement in asthma control.Clinical trial registered with www.clinicaltrials.gov (NCT 00214539).