Cyclooxygenase-2 (COX-2) is overexpressed in various types of human malignancies, including squamous cell carcinomas of the esophagus, but its clinicopathologic role in esophageal squamous cell carcinoma (ESCC) remains unclear. The aim of this study was to analyze expression of COX-2 in ESCC and to correlate this expression with clinicopathologic parameters and survival. From 1999 to 2003, endoscopic tissue samples from 110 patients with ESCC were collected for analysis. COX-2 expression was examined through immunohistochemical staining. Clinicopathologic data were analyzed to verify significance. COX-2 expression was detected in 50 of 110 ESCC specimens (45%). COX-2 expression was negative to weak in 73% (COX-2 low) and moderate to strong in 27% (COX-2 high) of tumors. Statistical differences between COX-2 high and COX-2 low were found according to status of the stage (stage IVM1a/IVM1b) (P=.001): cancer antigen (CA) 19-9 (normal/high) (P=.011), CA 12-5 (normal/high) (P=.011), and CA 15-3 (normal/high) (P=.035). Survival was significantly reduced among patients with high COX-2 expression (median overall survival, 3 mo) when compared with the COX-2 low group (median overall survival, 6 mo) (P=.0001). In the univariate analysis, age, body mass index, stage, COX-2, lactate dehydrogenase, CA 12-5, and CA 15-3 were significant factors for survival. With the use of COX regression analysis, only stage (P=.000), COX-2 (P=.000), lactate dehydrogenase (P=.023), and CA 15-3 (P=.002) were independent prognostic factors. Results showed that in patients with ESCC, COX-2 overexpression was significantly correlated with visceral metastases (IVM1b). COX-2 overexpression is an unfavorable prognostic factor in ESCC.