Two-hundred eighty-three boys with Duchenne dystrophy and 10 with Becker dystrophy have been followed for up to 10 years in a protocol that accurately measured their function, strength, contractures, and back curvature. Clinical heterogeneity is noted. Patients whose muscles were stronger were more likely to die from a cardiomyopathy. Weaker patients died from respiratory failure. A series of milestones is defined, which is of use in following the illness in an individual patient. This approach permits a scoring system that allows the severity of the disease to be defined in an individual boy. Evaluation of physical therapy and surgical intervention shows that night splints and scoliosis surgery are effective forms of treatment.