A protocol has been developed for the quantitative assessment of muscle function in children with muscle disease. It includes total muscle strength (% MRC) based on a clinical assessment of strength of 32 groups using the 6-point MRC grading; the force of 8 selected muscle groups measured with a specially designed electromyometer; a motor ability score based on 20 consecutive motor activities; walking times over 28 and 150 feet, and recording of muscle contractures. A 3-year sequential study of 61 boys with Duchenne dystrophy showed progressive decline of muscle strength with age, a close correlation of total strength and the motor ability score (r = 0.89), and a curvilinear relationship of muscle strength with walking times over 28 and 150 feet (r = 0.78 and 0.79, respectively). A profile of the natural progression of Duchenne dystrophy has been established which could serve as a reference base for the assessment of cases at varying ages and their response to therapy and management.