Osteoporosis causes significant morbidity for boys with Duchenne muscular dystrophy. Corticosteroid therapy given to prolong mobility may increase the rate of osteoporosis and risk of fracture. This study of 33 boys with Duchenne muscular dystrophy determined retrospectively the incidence of vertebral fractures particularly after initiation of corticosteroids. A latency period of 40 months after commencement of steroids occurred before the first vertebral fracture appeared. However, by 100 months of treatment approximately 75% had sustained a vertebral fracture.