Aims: It is currently accepted that primary ovarian tumours rarely, if ever, give rise to mucinous ascites/pseudomyxoma peritonei (PMP) which most commonly results from the intra-abdominal spread of an appendiceal mucinous neoplasm. However, primary ovarian mucinous tumours of appendiceal type arising within mature cystic teratomas appear to represent an exception to this rule. In this report two further examples of this rare tumour are described, and the immunohistological phenotype including expression of MUC proteins is compared with secondary ovarian involvement by low-grade appendiceal mucinous neoplasm.Methods: Two cases of ovarian mucinous tumour associated with mature cystic teratoma and PMP are described. The tumours were examined immunohistochemically for expression of cytokeratin (CK)7, CK20, carcinoembryonic antigen (CEA), CDX-2, MUC2, MUC5AC and MUC6. The results were compared with four cases of ovarian neoplasia secondary to primary appendiceal low-grade mucinous tumour.Results: The ovarian mucinous tumours associated with mature cystic teratomas were morphologically similar to those secondary to appendiceal neoplasia. They comprised irregularly distributed glands and cysts lined by tall, mucin-rich epithelial cells exhibiting focal villoglandular architecture and low grade cytological atypia. The immunophenotype of the teratoma-associated tumours and those secondary to appendiceal neoplasia was identical: there was strong and diffuse expression of CK20, CEA, CEX-2, MUC2 and MUC5AC with no reactivity for the other antisera tested.Conclusions: PMP associated with primary ovarian neoplasia is rare, and probably restricted to mucinous tumours arising in mature cystic teratomas. The immunohistological findings in this study further support the view that such tumours exhibit a lower gastrointestinal and, more specifically, appendiceal phenotype. Careful examination and sampling of the ovaries may be required to demonstrate the teratomatous component of these tumours.