Mucocele of the appendix is a rare lesion, characterized by distension of the lumen due to accumulation of mucoid substance. This disease is often asymptomatic and pre-operative diagnosis is rare. If untreated, one type of mucocele may rupture producing a potentially fatal entity known as pseudomyxoma peritonei. The type of surgical treatment is related to the dimensions and to histology of the mucocele. Appendectomy is used for simple mucocele or for cystadenoma. Right hemi-colectomy is recommended for cystadenocarcinoma. In this paper, we report a case of a 51-year-old woman with a mobile, painless mass in the right lower quadrant of abdomen caused by a giant appendiceal mucocele. Imaging showed a large, tubular, cystic structure extending below from the inferior wall of the cecum. Surgery revealed a giant retro-cecal appendix measuring 17 cm in length and 4 cm in diameter. The final pathologic diagnosis was mucocele caused by mucinous cystadenoma.