A rare case of peritonitis by cecal perforated diverticulum which was diagnosed after surgery by histology is reported. Preoperative diagnosis is often ignored and, in rare cases, is not always clear at laparotomy. Clinical aspects, diagnostic procedures, histological picture and surgical treatment are described. In the literature diverticula are classified as "solitary" and "multiple", "congenital" and "acquired", "true" and "false" and finally "usual type" and "hidden variant". The treatment of cecal acute diverticulitis is, in the majority of patients, surgery. Simple resection of the diverticulum and suture of the breach in double layer is the elective operation of non-complicated diverticulum or with an inflamed projection from the cecal wall ("usual type"). Segmental resection of colon or "right hemicolectomy" is recommended when a precise diagnosis is not possible and in complicated diverticula ("hidden variant"). On the basis of personal experience it is underlined that diagnosis is rarely made preoperatively since the clinical picture suggests an acute appendicitis. If there are some doubts for appendicitis, a incidental tumor or possible perforated diverticulum of the cecum should be considered and urgent surgery is mandatory.