Appendiceal mucocele is an uncommon disorder caused by accumulation of mucus within the appendiceal lumen. Mucoceles represent a heterogeneous group comprising various histopathologic lesions including mucosal hyperplasia, cystoadenomas, and cystoadenocarcinomas and prognosis is related to these subtypes. The most common symptom is pain or a palpable mass in the right lower quadrant on physical examination. The preoperative diagnosis is performed with abdominal U.S. and confirmed with CT scan; typical CT scan image is a capsulated cystic mass with calcification of the wall while U.S. pattern shows cystic lesion with the onion skin sign considered a specific sonographic marker for appendiceal mucocele. In conclusion a cystic mass sonographically detected with onion skin sign, in the presence of normal female reproductive organs, suggest the diagnosis of appendiceal mucocele.