Purpose: The objective of this study was to evaluate breast carcinomas for the expression of cyclooxygenase-2 (Cox-2) using a tissue microarray (TMA) and to determine its clinical and prognostic relevance. Methods: We analyzed Cox-2 expression in 600 samples from 200 breast carcinomas immunohistochemically performing TMA technology and semiquantitative analysis. Results were correlated with various clinicopathological variables and follow-up data. Expression of estrogen receptor, progesterone receptor, Ki-67, and Her-2/neu-oncogene was analyzed and correlated with Cox-2 status. Results: We observed a moderate or strong cytoplasmic staining for Cox-2 in 78 (40.6%) of breast carcinomas. Increased Cox-2 expression corresponded to higher pT stage (P=0.038), amplification of Her-2/neu (P=0.032), lymphovascular invasion (P=0.006), a high MIB-1 labeling index (LI) (P < 0.001), and histological grading (P=0.013). We also observed an inverse relationship between strong Cox-2 expression and estrogen and progesterone receptor content of tumors (P=0.037 and P=0.010). However, we could not demonstrate a significant association between Cox-2 staining and overall survival or disease free survival time. Conclusions: These results suggest that Cox-2 expression is significantly associated with less differentiated and more aggressive breast carcinomas and might therefore be a useful prognostic indicator as well as a target for therapy.