BACKGROUND Appendiceal neoplasms are rare. Patients may present with clinical symptoms suggestive of "acute appendicitis" or other unspecific abdominal complaints. An appendiceal mucinous neoplasia is sometimes diagnosed during a laparotomy performed on another indication. Frequently, the condition remains undiagnosed until the pathologists' examination. METHODS We present the case of a 49-year-old woman who presented with symptoms suggestive of acute appendicitis. An appendectomy was performed. The appendectomy specimen revealed a mucinous cystadenoma with a diverticulum in the appendix. The case is discussed in the light of the current literature. CONCLUSIONS Appendiceal tumours account for less than 0.4% of neoplasias in the gastrointestinal tract and are found in less than 1% of appendectomies. Mucinous lesions ("mucocele") are classified as mucosal hyperplasia, mucinous cystadenoma, or mucinous cystadenocarcinoma. However, there are reports of great variability in the biological behavior, especially concerning the development of pseudomyxoma peritonei. No current consensus exists as to diagnostic criteria or treatment. A radically removed appendix is curative in most cases of mucinous cystadenoma, whereas right hemicolectomy should be considered for patients with malignant mucinous lesions of the appendix.