**Abstract**

Aneurysmal bone cysts (ABCs) are vascular, rapidly expanding, and locally destructive cystic lesions. It is primarily found in childhood and adolescence. They typically involve the long tubular bones. Approximately 3–6% presented in the head and neck region, with the mandible being the most common site. Involvement of the ethmoid sinuses is extremely rare with only 13 cases reported in English literatures. Here we are presenting as an example an atypical case of an adolescent male patient, which had chief complaint of right-sided nasal blockage and right eye proptosis of 6-month duration. Local examination of the face showed right eye proptosis, with bulging around the right medial canthus and lateral wall of the nose. MRI revealed multiple fluid-fluid levels of varying signal intensities on T2-weighted image suggestive of aneurysmal bone cyst. Endoscopic sinus surgery was carried out to remove the swelling and tissue specimen sent for histopathology, which confirmed the diagnosis. Patient had smooth recovery after. Throughout this chapter, we will discuss aneurysmal bone cyst from its name, origin, and histopathology. Hence, the field of interest here is sino-nasal region, thorough discussion for ABC in sino-nasal region including its clinical presentation, how to reach into diagnosis, treatment method, and finally the prognosis and recurrence.

**Keywords:** aneurysmal bone cyst, ethmoid sinus, pediatric, fluid-fluid level
