The etiology is not entirely known, however viral infection in association with genetic susceptibility has been postulated. The majority (approximately three quarters) of patients are asymptomatic at the time of diagnosis. Presenting symptoms include: Localized pain and tenderness Increased focal temperature due to hyperaemia (due to hypervascularity) Increased bone size: historically changing hat size was a give-away Bowing deformities Kyphosis of the spine Decreased range of motion Signs and symptoms relating to complications Polyostotic disease is more common than the monostotic type 1. The most frequent sites of involvement are: Spine Pelvis (often asymmetric) Skull Proximal long bones