We present the case of a 21 year old male who developed an aneurysmal bone cyst (ABC) on a background of fibrous dysplasia (FD). He was diagnosed with FD aged 4 and has extensive disease with marked craniofacial involvement, including pituitary fossa. Past complications include a fractured right femur and right optic nerve neuropathy requiring decompression but with residual right visual loss. In 2013 he developed mild left optic nerve compression, treated with steroids. He self-treated a second episode in 2013. He has no hormonal abnormalities. He subsequently developed a rapidly expanding left frontal bone lesion with imaging suggesting an ABC. Given the rapid expansion and his compromised vision, he underwent a craniotomy with excision of an 8 9 10 cm lesion with minimal blood loss despite no preoperative arterial ablation. Histology showed an ABC arising from FD. Vision in the left eye is now completely normal. He has no evidence of ABC recurrence. FD is an osteoblast disorder in which bone is replaced by dysplastic fibrous tissue. FD is caused by a postzygotic activating mutation of the G-protein alpha-subunit. It can be monostotic or polyostotic, have overlying caf e-au-lait pigmentation, and may cause hormonal hypersecretion (McCune-Albright Syndrome). Malignant transformation, presenting with pain and an expanding mass, can occur, with polyostotic disease and previous radiation increasing risk. ABCs are rare benign lesions presenting with similar symptoms but distinct features on imaging. ABCs can be either primary or secondary to malignancies or FD. There are several theories for the pathogenesis of ABCs. Treatment is surgical with a high risk of intraoperative haemorrhage. The combination of craniofacial FD with secondary ABC is rare with limited cases in the literature. Our case is of a 21 year old male with FD who develops an ABC. We review the literature in regards to craniofacial FD, ABC, treatments and outcomes.