Cyclooxygenase-2 (COX-2) has been shown to be involved in multiple steps of carcinogenesis. In this study, we examined COX-2 expression in nasopharyngeal carcinoma (NPC). COX-2 mRNA analyzed by reverse-transcription polymerase chain reaction (RT-PCR) was detected in 66% (16 of 24) of tumor tissues. Western blot analysis demonstrated that COX-2 protein level was increased in tumor tissues and was correlated with the expression level of mRNA. Immunohistochemical study showed that COX-2 was predominantly detected in cancer cells, and the staining pattern was cytoplasmic. Ten histologically normal nasopharyngeal tissues obtained from nasopharyngeal hyperplasia were also investigated. We found that COX-2 mRNA was detectable in three tissues and the COX-2 protein level was very low. The frequency of COX-2 overexpression was significantly higher in patients of the N1-N3 group than in patients of the N0 group (P=0.006). Taken together, these data suggest that COX-2 is overexpressed and is associated with increased lymphatic invasion in NPC.