PURPOSE: The purpose of this work was to study the differences of cyclooxygenase (COX-2) expression between Korean and Caucasian patients with early-onset breast carcinoma by immunohistochemistry. The test were analyzed to find a correlation between COX-2 and other biomarkers including HER-2/neu amplification, because we previously reported that a significant difference had been found in the expression of HER-2/neu between the two races. Furthermore, we investigated prognostic significance of COX-2 in Korean patients. MATERIALS AND METHODS: Sixty Korean women who were diagnosed breast carcinoma at 45 years old or younger and 60 Caucasian women with breast carcinoma were selected for this study. The median age of both groups was 37 years and tumor sizes were distributed evenly between the two group. Paraffin embedded blocks of primary tumor were processed for immunohistochemical staining of COX-2. The COX-2 expression was evaluated according to the percentage of positive cells and the intensity of staining. And the results were compared with the data of the previous studies to find correlation between COX-2 and other parameters and survival data. RESULTS: Proportion of the COX-2 expression in total patients was 27.6%. The percentage of tumors that stained positive for COX-2 in Korean and Caucasian women with early-onset breast carcinoma were 37.9% and 20.8%, respectively. The difference was statistically not significant(p=0.090). Expression of COX-2 was not associated with several clinicopathologic parameters including HER-2/neu overexpression, but negative estrogen receptor status was correlated with significance (p=0.046). The 5 year disease free survival rate for patients with COX-2 expression was 67.9%, compared to 81.9% of the COX-2 negative patients and the result was statistically not significant. CONCLUSIONS: A significant difference was not found in the expression of COX-2 between the two groups of patients with early-onset breast carcinoma. And correlation between COX-2 and other parameters was not observed except estrogen receptor negativity. Large scaled further research including adiotherapy factors will be needed to identify COX-2 as a prognostic role in patients with early-onset breast carcinoma.