Background: Mucinous cystic neoplasms with ovarian stroma (OS) are rare cystic neoplasms in the liver and pancreas. The aim was to investigate whether OS in mucinous cystadenomas can originate from gonadal epithelium during early embryonic development. Patients and Methods: Pathology specimens of patients with mucinous cystadenomas were reviewed for OS. In human embryos, morphology of the peritoneal epithelium and the position of the gonads in relation to the embryonic liver, pancreas and spleen were examined. Results: From 1994 to 2004, 22 female patients presented with mucinous neoplasms of the liver or pancreas, including 19 cystadenomas and 3 cystadenocarcinomas. Mean age of the patients with cystadenoma in the liver was 44.8 years and with cystademona in the pancreas 41.2 years. OS was present in all mucinous cystadenomas, including the cystadenocarcinomas. In human embryos, preceding their 'descent', the gonads are situated directly under the diaphragm, dorsal to the liver, the tail of the pancreas and the spleen, but separated from these organs by the peritoneal cavity. In contrast to the peritoneal epithelium elsewhere, the cells covering the gonads show an activated morphology. Conclusion: OS in mucinous cystadenomas of the liver and pancreas suggest a common origin in epithelial cells that cover the embryonic gonads in early fetal life. Copyright (c) 2006 S. Karger AG, Basel.