An atypical case of acute appendicitis is reported, in which the initial presentation was not suggestive of this diagnosis. A routine work-up revealed elevated levels of inflammatory markers, an ultrasonographically normal abdomen and normal endoscopic findings in the upper gastrointestinal tract. During colonoscopy, performed 3 days after admission, the inflamed appendiceal orifice was localized and intubated using an endoscopic retrograde cholangio-pancreatography (ERCP) catheter; pus was aspirated. Elective appendectomy was carried out 5 months later on the asymptomatic patient. Histological analysis revealed fibrous thickening of the appendiceal wall, correlating with an old inflammatory process, along with changes typical for acute ulcerophlegmonous appendicitis. To the best of our knowledge, this is the first case of an endoscopically diagnosed and treated acute ulcero-phlegmonous appendicitis. We draw the following conclusions: (1) Acute appendicitis may present atypically, leading to an extensive clinical work-up including gastroscopy and colonoscopy. (2) This condition may be effectively treated, at least temporarily, by colonoscopic suction of appendiceal pus, provided that clinical urgency for laparotomy is absent. (3) Histological changes typically associated with acute ulcero-phlegmonous appendicitis may be present in asymptomatic patients and in patients without fever and with normal erythrocyte sedimentation rates and blood leucocyte counts. (4) Elective appendectomy should be considered in patients with conservatively healed acute appendicitis or periappendiceal abscess.