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Introduction: The definition of reliable outcome measures is of increasing interest in patients with Duchenne muscular dystrophy (DMD). Methods: In this retrospective study, we analyzed the longitudinal reliability of clinical and radiological endpoints in 29 ambulant patients with DMD. Clinical outcome measures included motor function measure (MFM) and timed function tests, while quantitative MRI data were mean fat fraction (MFF) and T2 relaxation time of thigh muscles. Statistical analysis was based on 3-, 6-, and 12-month follow-up data. Results: Quantitative MRI using the MFF was the most sensitive and powerful marker of disease progression with a sample size of four at 1-year follow-up, followed by the D1 domain of MFM (standing and transfer function) with a sample size of 12. Discussion: Our data support the longitudinal design of clinical trials over at least 12 months and the combinational use of clinical and radiological surrogate outcome measures. K E Y W O R D S clinical outcome measure, Duchenne muscular dystrophy, longitudinal analysis, motor function measure, quantitative muscle MRI, sample size