METHODS We report the case of a 24-year-old woman presenting with a rapidly growing mass in the right masseteric region. MRI suggested a diagnosis of a rhabdomyosarcoma but CT-scan evoked a benign lesion such as chondroblastoma or aneurismal bone cyst. The biopsy specimen proved the diagnosis of aneurismal bone cyst and the patient was treated by surgical curettage. There was no recurrence after 4 years of follow-up. CONCLUSIONS The aneurismal bone cyst of the jaw is a rare pseudocystic lesion, characterized by replacement of the normal bone with fibro-osseous tissue containing blood-filled sinusoidal or cavernous spaces. The lesion generally affects young persons under 20 years of age. The molar region, the angle, and the ascending ramus of the mandible are the predominantly affected sites. The lesion does not have any clinical or radiological specificity and surgical curettage is the standard treatment. Recurrence is rare and occurs within 30 months after surgery.