Appendicitis is a common and urgent surgical illness caused primarily by obstruction of the appendiceal lumen, but some other diseases can mimic this disease. A 42-year-old woman presented with a history of progressive abdominal pain over the right lower quadrant. Physical examinations implied acute appendicitis and appendectomy was performed. However, pathology revealed endometriosis. No malignancy was found and the patient was recommended for regular outpatient department follow-up.