A giant paediatric mandibular aneurysmal bone cyst and reconstruction with bilateral iliac bone graft. Aneurysmal bone cyst (ABC) is an unusual, non-neoplastic, expansile lesion of the bone. ABC is not a true cyst and it is characterised by the replacement of bone by fibro-osseous tissue containing blood-filled sinusoidal or cavernous spaces. The expanding mass often disrupts the normal bony architecture, with erosion of the cortex. Surgical treatment consists of complete excision or curettage.