A congenital, noninherited, benign anomaly of bone development in a single bone, it consists of the replacement of normal marrow and cancellous bone by immature bone with fibrous stroma. Monostotic fibrous dysplasia occurs with equal frequency in both sexes and normally develops early in life, with lesions frequently identified late in the first and early second decades. Most patients are asymptomatic, with the diagnosis often established after an incidental finding or with pain, swelling, or fracture.