OBJECTIVES: To study the responsiveness (sensitivity to change) of the Motor Function Measure (MFM) in detecting change in neuromuscular disease patients with the intent of using this measure in future clinical trials. DESIGN: Prospective cohort observational study. SETTING: Inpatient and outpatient facilities for follow-up and treatment of neuromuscular diseases. PARTICIPANTS: Patients (N=152) with various neuromuscular diseases aged 6 to 60 years. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURE(S): We used the MFM total score and its 3 subscores on 2 measurements grossly 1 year apart. The physicians and the patients (or proxy) were asked to provide their perceived change in functional status since the first MFM. These changes were expressed in 3 outcomes: deterioration, stability, or improvement. RESULTS: The overall 12-month-standardized mean change of the total score mean ± SD annual total score change was -2.4±5.5 points (P<.001), with patients with Duchenne muscular dystrophy (DMD) presenting the most significant change (-5.8±6.3, P<.001). The change in patients reporting deterioration (34%) was significantly larger than that of those reporting stability (47%) or improvement (10%) (-4.4±6.4 vs -2.0±5.6 and +0.9±4.4 points, respectively, P<.01). The 12-month-standardized total score changes were significantly greater in physician-rated deteriorated (49%) versus stable patients (51%), with mean differences in scores being -5.3±7.6 and -1.2±5.3, respectively (P<.001). CONCLUSIONS: The MFM showed a good responsiveness, especially in patients with DMD and agreements with patients' and physicians' perceived change. Confirming this responsiveness requires larger age groups of patients with DMD and other neuromuscular diseases as well as disease-specific interexamination delays.