Accumulating evidences indicate that cyclooxygenase (COX)-2 plays an important role in tumorigenesis in many human cancers. Yet the relationship between COX-2 and human renal cell carcinoma (RCC) remains unclear. The aim of our study was to evaluate COX-2 expression in human RCC cell lines and its role in tumorigenesis of human RCC. Among the human RCC cell lines (SMKT-R4, OS-RC-2, ACHN) and normal renal cell line RPTEC, COX-2 overexpression was found in OS-RC-2 cells both at mRNA and protein levels. COX-2 sense- and antisense-orientated vectors were constructed and transferred into RCC cells. Significant suppression of cellular proliferation was demonstrated in OS-RC-2 antisense transfectants, whereas promotion was found in SMKT-R4 sense transfectants by colony-forming assay despite the observation that COX-2 specific inhibitor NS398 exhibited similar IC50 among RCC cell lines by MTT assay. In comparison with parent cells and sense transfectants, significant suppression of COX-2 expression and PGE2 production and increase in butyrate-induced apoptosis were observed in OS-RC-2 antisense transfectants by Western blot, ELISA assay and FACS analysis, respectively. Furthermore, tumor growth and angiogenesis of OS-RC-2 antisense transfectants in nude mice was significantly suppressed and the survival time of these mice was significantly prolonged. Our study demonstrates that COX-2 is overexpressed in OS-RC-2 RCC cell line and plays an important role in tumorigenesis of the cells in vivo, which implies that COX-2 may be a therapeutic target for COX-2-expressing RCC, and that suppression of COX-2 expression by antisense-based strategy may have potential utility in treatment of COX-2-expressing RCC. © 2003 Wiley-Liss, Inc.