Increased CD4+CD25+Foxp3+ regulatory T cells (Tregs) predict poor prognosis in renal cell carcinoma (RCC). The aim of this study was to investigate the underlying causes of the aberrant accumulation of Tregs in RCC. pcDNA3.1-hCOX-2 and control pcDNA3.1 were transfected into the RCC cell line OS-RC-2. Under stimulation of anti-CD3/CD28 antibody and APC cells, isolated CD4+Foxp3- T cells were co-cultured with transfected OS-RC-2 culture medium supernatants and different control supernatants, respectively, and 96 h later, the proportion of Tregs in each group was detected using FACS. The suppressive ability of naturally isolated Tregs and transformed Tregs was also analyzed using [3H]-thymidine methods. The results showed that overexpression of COX-2 in OS-RC-2 cells led to higher expression of prostaglandin E2 (PGE2) in the culture medium supernatants. In addition, there was an apparent incremental increase in the percentage of Tregs in the CD4+Foxp3- T cells cultured with the COX-2-overexpressing OS-RC-2 culture medium supernatants. Furthermore, transformed Tregs had the same suppressive ability as naturally isolated Tregs. In summary, transfected RCC cell line culture medium supernatants were capable of converting CD4+Foxp3- T cells to Tregs by producing high levels of PGE2, while COX-2 inhibitors reduced the proportion of transformed Tregs in a dose-dependent manner. Thus, COX-2 inhibitors may induce a local anti-tumor effect and, in turn, may contribute to the eradication of RCC by decreasing transformed Tregs.