Mucinous cystadenomas of the appendix are rare tumours. In this article we describe the case of a young woman who presented with abdominal pain and a clinical pattern likely to a chronic appendicitis. At laparotomy we found a dilatation of the appendix which contained an abundant quantity of mucus. Histological examination showed a mucinous cystadenoma of the appendix. This neoplasm requires a surgical treatment, usually only appendectomy and right hemicolectomy in case of involvement of the caecum. During operation, the surgeon must take care in handling the appendix because of the risk of rupture with consequent dissemination of epithelial cells in the peritoneal cavity (pseudomyxoma peritonei).