This article presents the analysis of a rare clinical case of treatment of a patient with mucinar cystadenoma of the appendix. It has been demonstrated that the clinical course of this disease, as a rule, does not have a clear specific clinical picture, but most often it is clinically similar to chronic appendicitis, ovarian cysts, tumours of the abdominal cavity, and intestinal colic. Despite the progress in the radiological diagnosis of diseases of the abdominal organs, the diagnosis of mucocele of the appendix is extremely rare. The paper presents a case of treatment of a female patient who was admitted to M.V. Sklifosovsky Poltava Regional Clinical Hospital for emergency with suspected acute appendicitis. Following pre-operative examination, the patient was diagnosed as having “Acute appendicitis. Appendicular abscess”, and according to urgent indications the patient was operated on. The extent of surgical operation included appendectomy, drainage of the abdominal cavity. The macroscopic preparation was sent for a histopathological examination, where the diagnosis of "Mucinar cystadenoma of the vermiform appendix" was established. The patient was discharged from the surgical department in satisfactory condition on the 7th day. Considering that in every fifth patient, mucocele of the appendix is combined with synchronous or metachromic cancer of the colon, and in 4-24% of cases with cancer of the ovaries, gallbladder, thyroid and breast, the patient is further subject to strict examination and observation. Therefore, the problem of detecting mucocele of the appendix is relevant and requires additional investigation for diagnosis and differential diagnosis with other cystic formations of the abdominal organs.