Appendiceal mucocele is a rare disease. Sometimes it is discovered accidentally and sometimes it resembles acute appendicitis. Correct diagnosis before surgery is very important for the selection of adequate surgical treatment to avoid intraoperative and postoperative complications. Ultrasonography, and particularly computed tomography, should be used extensively for this purpose. If mucocele is treated incorrectly pseudomyxoma peritonei, which is characterized by malignant process, may develop. We present a case of a 54-year-old man who was admitted to the emergency department with the signs of acute appendicitis. Open surgery was performed. At the time of surgery, a cystic mass of the appendix with dimensions 7 3 4 33 cm, with inflamed walls, but without perforation was discovered in the right iliac fossa. No discharge was found in the peritoneal cavity. Diagnosis of mucocele was suspected. Only appendectomy was performed because no pathologic process was found in the base of the appendix and lymph nodes were not increased in size. Hystopathologic diagnosis was mucinous cystadenoma. After 2 years, the patient is feeling well.