A 13-year-old girl presented with a two-year history of pain in the right thigh and right forearm. Engelmann's disease was diagnosed on the basis of radiological appearances and histological examination of the bone. Her symptoms subsided after biopsy of the bone, but two weeks later she developed severe pain in the left tibia and was unable to walk. Radiological features of Engelmann's disease were found in the left tibia and other long bones although these bones had been radiologically normal one month previously. Treatment with prednisolone gave rapid relief of pain and allowed the patient to become mobile again. Four months later the radiological appearances showed significant improvement. The use of corticosteroids in this disorder is discussed.