OBJECTIVE: To validate a useful version of the Motor Function Measure (MFM) in children with neuromuscular diseases aged <7 years old. DESIGN: Two prospective cohort studies that documented the MFM completion of children aged between 2 and 7 years old. SETTING: French-speaking rehabilitation departments from France, Belgium, and Switzerland. PARTICIPANTS: Healthy children (n=194) and children with a neuromuscular disease (n=88). INTERVENTIONS: Patients were rated by the MFM either once or twice by trained medical professionals, with a delay between the 2 MFMs ranging between 8 and 30 days. MAIN OUTCOME MEASURE: Intra- and interrater reliability of the MFM. RESULTS: The subtests making up the MFM-32, a scale monitoring severity and progression of motor function in patients with a neuromuscular disease in 3 functional domains, were carried out in healthy children aged 2 to 7 years. Twenty items of the MFM-32 were successfully completed by these children and were used to constitute the MFM-20. Principal component analysis of the MFM-20 confirmed the 3 functional domains. Inter- and intrarater reliability of the 3 subscores and total score were high (intraclass correlation coefficient >.90), and discriminant validity was good. CONCLUSIONS: The MFM-20 can be used as an outcome measure for assessment of motor function in young children with neuromuscular disease.