Background : Oncogene expression in Paget's disease of the breast is not well known. To characterize invasive ductal carcinoma associated with Paget's disease, we studied expres-sion of anaphase promoting complex (APC) with its regulatory proteins. Methods : Immuno-histochemical stainings were done with 10 cases of invasive ductal carcinoma associated with Paget's disease for APC, pituitary tumor transforming gene (PTTG), cyclin B1, p53, cyclin D1, and c-erbB-2. The expressions of these markers in Paget's disease were compared with those in the associated with carcinoma. Results : APC, PTTG, cyclin B1, and c-erbB-2 were positive in all of the cases with both Paget's disease and underlying carcinoma. p53 was expressed in Paget's disease of 6 cases (60%) and in carcinoma of 7 cases (70%). Cyclin D1 was positive in Paget's disease of 8 cases (80%) and in carcinoma of 9 cases (90%). Conclusions : Breast carcinomas with Paget's disease seem to be distinguished by the high expression of APC, cyclin B1, PTTG, c-erbB2, and cyclin D1 in contrast to breast can-cers without Paget's disease. Furthermore, the similar expression pattems of APC and APC regulatory proteins in both Paget’s disease and underlying breast cancer support the epider-motropic theory as its pathogenetic mechanism.