The aim of this study is to report the experience of a case of benign multicystic mesothelioma of the peritoneum presented with acute appendicitis symptomatology. A 28 years old man with right lower and upper abdominal pain was admitted into hospital. Because of the clinical picture, the symptomatology and the leukocytosis a diagnosis of acute appendicitis was made and the patient underwent appendicectomy according to Mc Burney. At laparotomy some cc of purulent fluid were sucked and a cystic mass that contained clear fluid was revealed. In consequence of the incidental diagnosis a following middle laparotomy was made with a careful surgical excision of the mass and of the appendix. Macroscopically the lesion was identified like a neoplastic mass 25 centimeters in diameter, with a multicystic and fibrous-adipose aspect, with cysts 5 centimeters in diameter. The cystic spaces were lined by a layer of eptelial cells which presented positive reaction for cytokeratin and EMA, whereas endothelium markers were absent. The ultrastructural, morphological and immunohistochemical findings were diagnostic of a benign multicystic mesothelioma.