Summary Background Cardiorespiratory failure is the leading cause of death in Duchenne muscular dystrophy. Based on preclinical and phase 2 evidence, we assessed the efficacy and safety of idebenone in young patients with Duchenne muscular dystrophy who were not taking concomitant glucocorticoids. Methods In a multicentre phase 3 trial in Belgium, Germany, the Netherlands, Switzerland, France, Sweden, Austria, Italy, Spain, and the USA, patients (age 10–18 years old) with Duchenne muscular dystrophy were randomly assigned in a one-to-one ratio with a central interactive web response system with a permuted block design with four patients per block to receive idebenone (300 mg three times a day) or matching placebo orally for 52 weeks. Study personnel and patients were masked to treatment assignment. The primary endpoint was change in peak expiratory flow (PEF) as percentage predicted (PEF%p) from baseline to week 52, measured with spirometry. Analysis was by intention to treat (ITT) and a modified ITT (mITT), which was prospectively defined to exclude patients with at least 20% difference in the yearly change in PEF%p, measured with hospital-based and weekly home-based spirometry. This study is registered with ClinicalTrials.gov , number NCT01027884. Findings 31 patients in the idebenone group and 33 in the placebo group comprised the ITT population, and 30 and 27 comprised the mITT population. Idebenone significantly attenuated the fall in PEF%p from baseline to week 52 in the mITT (−3·05%p 95% CI −7·08 to 0·97, p=0·134, vs placebo −9·01%p –13·18 to −4·84, p=0·0001; difference 5·96%p 0·16 to 11·76, p=0·044) and ITT populations (−2·57%p –6·68 to 1·54, p=0·215, vs −8·84%p –12·73 to −4·95, p<0·0001; difference 6·27%p 0·61 to 11·93, p=0·031). Idebenone also had a significant effect on PEF (L/min), weekly home-based PEF, FVC, and FEV1 . The effect of idebenone on respiratory function outcomes was similar between patients with previous corticosteroid use and steroid-naive patients. Treatment with idebenone was safe and well tolerated with adverse event rates were similar in both groups. Nasopharyngitis and headache were the most common adverse events (idebenone, eight 25% and six 19% of 32 patients; placebo, nine 26% and seven 21% of 34 patients). Transient and mild diarrhoea was more common in the idebenone group than in the placebo group (eight 25% vs four 12% patients). Interpretation Idebenone reduced the loss of respiratory function and represents a new treatment option for patients with Duchenne muscular dystrophy. Funding Santhera Pharmaceuticals.