Mucinous adenocarcinoma of the appendix is a very rare, slow growing, well-differentiated epithelial neoplasm of the appendix characterized by abundant mucin production. Clinically, it presents as acute appendicitis (with abdominal pain, fever, leukocytosis) or as pseudomyxoma peritonei (wide-spread presence of mucin within the peritoneal cavity), however some patients may be completely asymptomatic at the time of diagnosis. In many cases, a second gastrointestinal malignancy is present. The disease is mucinous adenocarcinoma of the appendix (Monarch Disease Ontology entry MONDO_0018330). Also known as: appendiceal mucinous adenocarcinoma, appendix mucinous adenocarcinoma, vermiform appendix mucinous adenocarcinoma.