The authors report case of mucinous cysto-adenocarcinoma of the vermiform appendix which was not diagnosed during appendectomy but by routine histological tests. The patients was reoperated and underwent right hemicolectomy with lymphadenectomy, ending with the creation of an ileo-transverse stomy. At the follow-up after 1, 6 and 12 months, the patient showed no signs of recurrence of the disease. The authors examine this rare pathology which, owing to the difficulty of preoperative diagnosis, often results in the patient undergoing a second operation, and given the early diffusion of the disease frequently presents a severe prognosis.