The aim of this study was to investigate protein expression of adenomatous polyposis coli (APC) in primary and metastatic serous ovarian carcinoma. The expression of beta-catenin and E-cadherin was additionally analyzed. One hundred and thirteen primary (n = 56) and metastatic (n = 57) lesions were immunohistochemically stained for APC, E-cadherin, and beta-catenin. Staining extent was scored. Possible differences in immunoreactivity in primary and metastatic sites and the association between the proteins analyzed were evaluated statistically. Cytoplasmic immunoreactivity for APC was found in 67/113 (59%) tumors, most often in the majority (> 50%) of cells. E-cadherin was detected in 102/113 (90%) carcinomas, while beta-catenin was expressed in 109/113 (97%) specimens. Nuclear expression of beta-catenin was seen in 3/113 (3%) specimens, all negative for APC. APC and beta-catenin were often coexpressed, but this finding failed to reach statistical significance (p = 0.11). A significant association was seen between E-cadherin and beta-catenin expression (p = 0.001). APC expression was comparable in primary and metastatic tumors (p > 0.05). In conclusion, APC expression is absent in a considerable number of both primary and metastatic ovarian carcinomas, but this finding is only rarely coupled to nuclear accumulation of beta-catenin. These findings support the role for beta-catenin signaling via the Wingless/Wnt pathway in ovarian carcinoma. The mechanism behind the down-regulated expression of APC in serous ovarian carcinoma and its significance has yet to be elucidated.