Progressive diaphyseal dysplasia (PDD), a rare disorder of bones, in recent years has been accepted as a systemic disease within the spectrum of connective tissue disorders associated with immunological abnormalities. Steroids have been used in the treatment of PDD with variable success. In this report PDD is described in a 5-year-old boy who presented with leg pain, fatigue, headache and anorexia with an onset in infancy. Physical examination revealed a waddling gait, thorax deformity and thickening in the upper extremities. The diagnosis was made by radiologic demonstration of cortical thickening and a narrowed medullary cavity of the long bones of extremities. Bone scintigraphy showed areas of increased osteoblastic activity in the diaphyseal part of the long bones of extremities and the skull. Electron microscopic examination revealed myopathic and vascular changes. Serum immunoglobulin A, G and M levels were elevated and CD4 positive T cell numbers were low. Deflazacort, a steroid with a similar anti-inflammatory effect to prednisolone but with fewer adverse effects, was started in a dose of 1.2 mg/kg/day. Deflazacort treatment resulted in clinical and radiological improvement within 12 months with no side effects. In conclusion, steroids may be recommended as an effective method of treatment in PDD and deflazacort may be a safe alternative steroid.