Background/Aims: An intraoperative diagnosis in cases with primary and metastatic ovarian carcinomas is important for adequate treatment. The aim of the present study was to find a reliable method to discriminate primary from metastatic mucinous adenocarcinomas (MACs) of the ovary intraoperatively. Methods: Clinical features of all primary and metastatic mucinous ovarian carcinomas diagnosed from 1994 to 2008 at the Osaka University and Osaka Rosai Hospitals, Osaka, Japan, were reviewed retrospectively. Results: Among the 73 MACs, 51 (70%) and 22 cases (30%) were diagnosed as primary and metastatic ovarian carcinomas, respectively. The distributions of tumor size, patient age, and serum CA125 level were significantly different between primary and metastatic cases. Our algorithm that categorizes patients 6 50 years whose tumor size was < 10 cm into metastatic cases, and patients < 50 years, or the ones whose tumor size was 6 10 cm, into primary tumors, provided an 84% accuracy in our population. Conclusions: A more reliable method to discriminate primary from metastatic MACs of the ovary by patient age and serum tumor markers was derived from our study population. Copyright (C) 2011 S. Karger AG, Basel