Purpose: To determine the effect of cyclooxygenase (COX)-2 expression on clinical behavior in irradiated and nonirradiated rectal carcinomas. Experimental Design:Tumor samples were collected from 1,231patients of the DutchTME trial, in which rectal cancer patients were treated with standardized surgery and randomized for preoperative short-term (5 Â 5 Gy) radiotherapy or no preoperative radiotherapy. Tissue microarrays were constructed from primary tumor material, and COX-2 expression was assessed by immunohistochemistry. Tumor cell apoptosis was determined by M30 immunostaining. Results: A high level of COX-2 expression after radiotherapy was associated with low levels of tumor cell apoptosis (P = 0.001). COX-2 expression had no significant effect on patient survival or tumor recurrence in nonirradiated tumors. However, in patients receiving preoperative radiotherapy, high level of COX-2 expression was associated with higher incidence of distant recurrences [P = 0.003; hazard ratio (HR), 1.7; 95% confidence interval (95% CI), 1.2-2.5] and shorter disease-free survival (P = 0.002; HR, 1.8; 95% CI, 1.2-2.5) and overall survival (P = 0.009; HR, 1.5; 95% CI, 1.1-2.0), independent of patient age, tumor stage, tumor location, or the presence of tumor cells in the circumferential resection margin. Conclusions: A high level of COX-2 expression after preoperative radiotherapy in resection specimens is associated with apoptosis resistance, high distant recurrence rates, and a poor prognosis in rectal cancer.In recent years, the role of a key enzyme in prostaglandin synthesis, cyclooxygenase (COX)-2, has been appreciated in cancer development and progression. COX-2 is responsible for the conversion of arachidonic acid to prostaglandins and other eicosanoids. In addition to its well-known role in inflammatory reactions, COX-2 plays a role in tumor progression, angiogenesis, metastatis, and abrogation of the antitumor immune response (1 -4). COX-2 prevents apoptosis by generation of antiapoptotic PGE 2 (5) and PGI 2 (6) and by removal of the proapoptotic substrate arachidonic acid (7). PGE 2 induces transformations that result in increased Bcl-2 expression and prolong the cell cycle G 1 phase with increased cyclin D1 expression (8). Numerous epidemiologic studies have indicated that the use of nonsteroidal antiinflammatory drugs, and COX-2 inhibitors is associated with a significant decreased incidence and mortality rate in colorectal cancer (9 -12). In addition, selective COX-2 inhibitors have been shown to decrease COX-2 expression and COX-2 activity in gastrointestinal malignancies (13). The clinical effect of COX-2 expression has been evaluated in a large number of studies in colorectal cancer and results have not been consistent (9,14,15). Considering the distinct differences in tumor biology (16), treatment, recurrence rates, and metastatic behavior, it is regrettable that most studies make no distinction between rectal and colon cancer.The purpose of the current study was to obtain a conclusive answer of the...