Cyclooxygenase-2 (COX-2) is overexpressed in various types of human malignancies including squamous cell carcinomas (SCCs) of the esophagus, but little is known about COX-2 expression in premalignant esophageal squamous dysplasia. To elucidate the role of COX-2 in esophageal carcinogenesis, we examined the expression of this enzyme in normal squamous epithelium (n = 42), squamous dysplasia [high-grade dysplasia (HGD, n = 41; low-grade dysplasia (LGD, n = 33)]; carcinoma in situ (n = 16), mucosal invasive carcinoma (n = 18), and advanced SCC (n = 45). Immunohistochemistry showed a significantly high COX-2 expression in HGD compared with other lesions. The COX-2 score, an index determined by intensity and positivity of COX-2 staining (maximum 3.0), was 0.29 +/- 0.04 in normal esophagus, 1.75 +/- 0.11 in LGD, 2.89 +/- 0.05 in HGD, 2.17 +/-0.18 in CIS, 1.95 +/- 0.22 in mucosal invasive carcinoma, and 1.81 +/- 0.08 in advanced SCC. Results of reverse transcription-PCR assays confirmed those obtained by immunohistochemistry. COX-2 expression correlated with proliferation activity assessed by the proliferating cell nuclear antigen index in dysplastic lesions (P = 0.001) but not in SCCs. COX-2 expression in SCC did not correlate with various clinicopathological parameters including prognosis. Our results indicate that COX-2 is a sensitive marker for HGD and suggest that COX-2 may be involved in early stages of squamous carcinogenesis of the esophagus.