Endometriosisis an estrogen-dependent inflammatory disease which occurs in 5-50% of women in the reproductive period. May be asymptomatic or can cause extensive gynecological disorders, from pelvic pain to infertility in 45% of patients. The patient, aged 23, was admitted to hospital Surgery Clinic, because of pain in the lower right part of the abdomen, with a high temperature of 39 degrees, pain and disgust, without vomiting. Pain was present last 4 days, and the other symptoms have occurred in the last 24 hours. Clinical examination, painful sensitivity was found with abdominal tightness, particularly pronounced in the lower right quadrant of the abdomen. Findings of rectal and vaginal examination were normal. Laboratory findings were in normal range, except for C-reactive protein in the presence of leukocytosis. Computed tomography of the abdomen it was noticed, retrocecal, enlarged appendix diameter 20 mm. Laparotomy, under general endotracheal anesthesia were identified, retrocecal, oedematous- enlarged appendix, dark brown in color, with no signs of peritonitis. Right ovary and the terminal portion of the small intestine were macroscopically visible without macroscopic changes. Appendectomy was performed, and the operative material was sent for histopathological analysis. Standard microscopic examination with immunohistochemical analysis confirmed the diagnosis of acute appendicitis with endometriosis. The postoperative course was regular and the patient was discharged from hospital on the fifth day in a good general condition. Endometriosis of the appendix is very rare and can be a cause of acute appendicitis. Preoperative diagnosis is almost impossible, and the importance of the precise histological analysis is particularly high among women treated for infertility.