The purpose of this study was to evaluate cyclooxygenase-2 (COX-2) expression in the successive steps of breast carcinogenesis and to determine its correlation with HER-2/neu and p53 expression in invasive ductal carcinomas of the breast. Immunohistochemical staining with anti-COX-2 antibody was performed in normal breast tissue, usual hyperplasia, ductal carcinoma in situ, and invasive ductal carcinoma. Expression of COX-2 in invasive ductal carcinoma was correlated with immunohistochemical expression of HER-2/neu and p53 protein. COX-2 expression was found to be progressively elevated along the continuum from normal breast tissue to invasive ductal carcinoma (P<0.001). COX-2 expression significantly correlated with p53 and HER-2/neu protein expression (P<0.05 and P<0.001). On multivariate analysis, only TNM stage and elevated COX-2 expression correlated with survival. Our results suggest that COX-2 may be involved in the carcinogenesis of the breast and may be an independent prognostic indicator in patients with invasive ductal carcinoma. HER-2/neu and p53 are likely to be involved in the regulation of COX-2 expression in invasive ductal carcinomas of the breast.