PURPOSE: It is well known that cyclooxygenase-2 (COX-2) is associated with carcinogenesis in many human cancers. In thyroid cancers, COX-2 expression in aging patients is known as a powerful prognostic factor of differentiated thyroid cancer. Therefore, in this study, we investigated the expression of COX-2 in cases of papillary thyroid cancer (PTC), as well as its association with other prognostic factors of thyroid cancer. METHODS: A total of 213 cases of papillary thyroid cancer were enrolled in this study and their clinicopathological characteristics were investigated retrospectively by a review of medicalrecords. Immunohistochemical staining for COX-2 was performed in the malignant tissue of the cases and the resultsof were then evaluated to determine if they were associated with other prognostic factors. RESULTS: The mean age of patients was 46 years (Range: 17~77 years old) and the tumor sizesranged from 0.1 to 5 cm (mean 1.2±0.9 cm). The pathological findings were as follows; thyroid capsule invasion in 101 patients (47%), lymph node metastasis in 76 patients (36%), multiplicity in 47 patients (22%), and bilaterality in 36 patients (17%). Positive results of immunohistochemical staining for COX-2 were noted in 131 patients (62%), however, the statistical analyses showed no significant association between COX-2 expression and other prognostic factors of papillary thyroid cancer. CONCLUSION: No association of COX-2 expression and prognostic factors of papillary thyroid cancer were found in this study. However, a larger prospectivestudy with survival analyses would provide a better understanding of thesignificance of COX-2 expression in cases of papillary thyroid cancer.