Background Lebrikizumab is a monoclonal antibody that modulates activity of interleukin‐13. The Phase 3 ACOUSTICS study assessed lebrikizumab efficacy and safety in adolescents with uncontrolled asthma despite standard‐of‐care treatment. Methods Adolescents (aged 12–17 years) with uncontrolled asthma, prebronchodilator forced expiratory volume in 1 s 40%–90% predicted, and stable background therapy were randomised 1:1:1 to receive lebrikizumab 125 or 37.5 mg or placebo subcutaneously once every 4 weeks. Primary efficacy endpoint was asthma exacerbation rate over 52 weeks. Results Between August 2013 and July 2016, 579 patients were screened and 346 were randomised; 224 (65%) completed the study with 52 weeks of treatment. Lebrikizumab 125 mg (n = 116) reduced the exacerbation rate at 52 weeks versus placebo (n = 117; adjusted rate ratio RR 0.49 95% CI 0.28–0.83; 51% rate reduction). Lebrikizumab 37.5 mg (n = 113) was less effective at reducing exacerbations (RR 0.60 95% CI 0.35–1.03; 40% rate reduction). In patients with blood eosinophil counts ≥300 cells/μl, both lebrikizumab doses reduced exacerbations (125 mg: RR 0.44 95% CI 0.21–0.89; 37.5 mg: 0.42 95% CI 0.19–0.93). Treatment‐emergent adverse events, serious adverse events, and adverse events leading to study discontinuation occurred in 155 (68%), 7 (3%), and 5 (2%) of 229 patients who received lebrikizumab (both 125 and 37.5 mg doses) and in 72 (62%), 4 (3%), and 1 (1%) of 117 who received placebo, respectively. No deaths occurred. Conclusion Lebrikizumab 125 mg reduced asthma exacerbation rates in adolescents with uncontrolled asthma. However, the study was prematurely terminated (sponsor's decision) potentially limiting interpretation of results. Clinical trial registration NCT01875003 (www.ClinicalTrials.gov).