The prognostic and predictive role of cyclo-oxygenase-2 (COX2) in breast cancer is still debated, and in particular, its role as a target of COX2 inhibitor (celecoxib) in neoadjuvant setting. We analyzed a series of 156 breast cancer samples from patients of the COX2 inhibitor-treated arm included in the REMAGUS-02 randomized phase II trial. COX2 gene expression was assessed by reverse transcription and quantitative polymerase chain reaction using ribonucleic acid from frozen biopsies. Pathological complete response (pCR) was the surrogate end-point. Significantly higher rates of grade 3, and estrogen and progesterone receptor negativity were observed in tumors with the highest expression of COX2. pCR rates were significantly higher in COX2-overexpressing tumors in patients receiving celecoxib. The test for interaction between COX2 gene expression and the celecoxib effect was statistically significant (p<0.01), but was not retained in the multivariate analysis. COX2 overexpression is predictive of pCR in patients with celecoxib-treated tumors. The efficacy of celecoxib in breast cancer might be improved by quantification of COX2 gene expression.