Human cardiac fibroblasts are the main source of cardiac fibrosis associated with cardiac hypertrophy and heart failure. Transforming growth factor-β1 (TGF-β1) irreversibly converts fibroblasts into pathological myofibroblasts, which express smooth muscle α-actin (SM α-actin) de novo and produce extracellular matrix. We hypothesized that TGF-β1–stimulated conversion of fibroblasts to myofibroblasts requires reactive oxygen species derived from NAD(P)H oxidases (Nox). We found that TGF-β1 potently upregulates the contractile marker SM α-actin mRNA (7.5±0.8-fold versus control). To determine whether Nox enzymes are involved, we first performed quantitative real time polymerase chain reaction and found that Nox5 and Nox4 are abundantly expressed in cardiac fibroblasts, whereas Nox1 and Nox2 are barely detectable. On stimulation with TGF-β1, Nox4 mRNA is dramatically upregulated by 16.2±0.8-fold (n=3, P<0.005), whereas Nox5 is downregulated. Small interference RNA against Nox4 downregulates Nox4 mRNA by 80±5%, inhibits NADPH-driven superoxide production in response to TGF-β1 by 65±7%, and reduces TGF-β1–induced expression of SM α-actin by 95±2% (n=6, P<0.05). Because activation of small mothers against decapentaplegic (Smads) 2/3 is critical for myofibroblast conversion in response to TGF-β1, we also determined whether Nox4 affects Smad 2/3 phosphorylation. Depletion of Nox4 but not Nox5 inhibits baseline and TGF-β1 stimulation of Smad 2/3 phosphorylation by 75±5% and 68±3%, respectively (n=7, P<0.0001). We conclude that Nox 4 mediates TGF-β1–induced conversion of fibroblasts to myofibroblasts by regulating Smad 2/3 activation. Thus, Nox4 may play a critical role in the pathological activation of cardiac fibroblasts in cardiac fibrosis associated with human heart failure.