The authors present the case of an 11-year-old boy with a painful, rapidly expanding lesion in the posterior triangle of the neck. There was no history of cervical trauma. Computerized tomography of the neck revealed a unicameral (single-chambered) aneurysmal bone cyst involving the C3 vertebra. Treatment was by open resection and curettage; no recurrence was seen at six months. We discuss the natural history, differential diagnosis, radiographic appearance and treatment modalities for this unusual, benign, expanding, osteolytic lesion containing blood-filled cystic cavities.