E-cadherin (E-CD), a Ca2+ -dependent adhesion molecule, plays a major role in the maintenance of intercellular junctions in normal epithelial cells in most organs. Recently, a correlation has been observed between a loss of E-CD and increased invasiveness of neoplastic cells. In this study, E-CD expression in the breast carcinoma was investigated using monoclonal antibody, anti-E-CD by immunohistochemical method. Expression of E-CD were evaluated in 57 breast carcinomas and correlated with their tumor grade, lymph node involvement, and hormonal receptor status. Histological types included in this study were 54 invasive ductal carcinomas (IDCs) of otherwise not specified and 3 invasive lobular carcinomas. Cases of histologic grade I IDC were 6, grade II 30, and grade III 18. Of 54 IDCs 39 (72.2%) showed moderate to strong linear staining at the cell borders regardless of their histologic grade, status of lymph node metastasis, and status of hormone receptor. Staining intensity of E-CD was reduced in 54 cases (83%) of IDC when compared with that of normal or benign breast lesions (P<0.01). All seven cases of intraductal carcinoma, which were included in 54 IDCs showed one or two grade reduced expression of E-CD than that of infiltrative lesions. Three invasive lobular carcinomas showed strong (1 case), moderate (1 case), and negative reactivity (1 case). The data indicated that loss of E-CD expression is a crucial event in the development of breast carcinoma.