Cystic fibrosis is an hereditary disease in which abnormally viscous secretion of exocrine glands causes chronic pulmonary infections and gastrointestinal disorders. The authors report the case of a patient with cystic fibrosis and appendicitis complicated by appendico-vesical fistula. The appendico-vesical fistula has been described during appendicitis; its association with the cystic fibrosis however has never been reported in the literature.