Radiotherapy is one of the principal modalities of rectal cancer treatment, and the ability to predict radio resistance could potentially improve survival through a targeted treatment approach. Cyclooxygenase-2 (COX-2) may protect against damage by irradiation that would justify the use of COX-2 inhibitors. The purpose of this study was to investigate the potential role of COX-2 in tumor response and outcome of patients with rectal cancer treated preoperatively with radiotherapy. Using immunohistochemistry, we examined COX-2 expression in 88 surgical specimens of rectal cancer treated preoperatively and in 26 pretherapeutic biopsies. We tested whether COX-2 expression was correlated with clinico-pathologic parameters and with survival and local recurrence. COX-2 was expressed in 50% of the pretherapeutic tumor biopsies and in 88.6% of post-irradiated surgical samples. COX-2 expression was correlated only with enhanced tumor inflammation (p = 0.03) and with tumor volume exceeding 30 cc (p = 0.05). COX-2 was not significantly correlated with patient survival, but none of the patients with COX-2 negative tumors did recur locally, whereas 80% of patients with local recurrences have COX-2 positive tumors. We conclude that COX-2 expression is overexpressed in the majority of rectal cancers treated with radiotherapy and likely plays a role in local relapse.