The mucocele of the appendix is the expansion of the appendix with the accumulation of a large amount of mucus. The mechanism and causes of mucocele are not fully understood. According to some authors, such changes in the appendix can occur due to cicatricial narrowing of the lumen of the appendix, compression or blockage of its base. Other authors believe that the mucocele of the appendix is a benign tumor that develops from the remnants of primitive mesenchyme and is sometimes prone to malignancy. Clinical manifestations of mucocele of the appendix are nonspecific. In a number of patients, this disease causes pain in the right abdomen, more often pulling, intermittent. However, the disease is often asymptomatic. In this regard, diagnosis is established only during performing an operation, most often, regarding acute appendicitis. Nevertheless, instrumental diagnostic methods such as ultrasound and computed tomography of the abdominal and pelvic organs make it possible to suspect mucocele. Despite the frequent asymptomatic, non-aggressive course, a number of life-threatening complications can become the outcome of the mucocele of the vermiform appendix. The most formidable complication is the rupture of the appendix with mucus entering free abdominal cavity, followed by the development of peritoneal pseudomyxoma due to implantation of mucus-forming cells. The only option for radical treatment of the mucocele of the appendix is a surgical intervention. A presented clinical case demonstrates the difficulties of diagnosis, as well as the features of surgical treatment of a patient with a mucocele of the appendix.