BACKGROUND: The available clinical outcome measures of disability in multiple sclerosis are not adequately responsive or sensitive. OBJECTIVE: To investigate the feasibility of inertial sensor-based gait analysis in multiple sclerosis. METHODS: A cross-sectional study of 80 multiple sclerosis patients and 50 healthy controls was performed. Lower-limb kinematics was evaluated by using a commercially available magnetic inertial measurement unit system. Mean and standard deviation of range of motion (mROM, sROM) for each joint of lower limbs were calculated in one minute walking test. A motor performance index (E) defined as the sum of sROMs was proposed. RESULTS: We established two novel observer-independent measures of disability. Hip mROM was extremely sensitive in measuring lower limb motor impairment, being correlated with muscle strength and also altered in patients without clinically detectable disability. On the other hand, E index discriminated patients according to disability, being altered only in patients with moderate and severe disability, regardless of walking speed. It was strongly correlated with fatigue and patient-perceived health status. CONCLUSIONS: Inertial sensor-based gait analysis is feasible and can detect clinical and subclinical disability in multiple sclerosis.