Purpose: Kidney cancer is notoriously chemo-resistant and abundantly expresses the Nox4 NADPH oxidase. To determine if Nox4 superoxide generation contributes to drug resistance, we assayed in vitro drug cytotoxicity following Nox4 shRNA silencing in human renal cancer cells. Materials and Methods: Human conventional kidney cell lines, 786-0 and RCC4 expressing Nox4-specific shRNA or a non-targeting, control shRNA were grown in serial dilutions of cisplatin, vincristine, doxorubicin, or etoposide. Cell viability curves were generated and the concentration required to kill 50% of the cells (IC₅₀) calculated for each drug. Apopotosis was estimated by TUNEL assay. Quantitative RTPCR and Western blots were used to confirm Nox4 silencing and evaluate expression of apoptotic pathway proteins. Results: Silencing significantly lowered the IC₅₀ for cisplatin, vincristine and etoposide, and promoted druginduced apoptosis by TUNEL assay. Improved sensitivity to cisplatin was reproduced by Nox inhibiton with diphenyliodonium, whereas induction of intracellular superoxide by dithiothreitol superoxide enhanced chemo-resistance. RT-PCR and Western blot revealed decreased expression of anti-apoptotic Bcl-XL and Bcl-2 and increased expression of pro-apoptotic Bax following Nox4 knockdown. Conclusion: Nox4 contributesto RCC chemo-resistance through modulation of pro-apoptotic and anti-apoptotic signaling, suggesting that Nox4 inhibition might enhance the efficacy of conventional cytotoxic drugs against RCC.