Protein misfolding in the endoplasmic reticulum (ER) contributes to the pathogenesis of many diseases. Although oxidative stress can disrupt protein folding, how protein misfolding and oxidative stress impact each other has not been explored. We have analyzed expression of coagulation factor VIII (FVIII), the protein deficient in hemophilia A, to elucidate the relationship between protein misfolding and oxidative stress. Newly synthesized FVIII misfolds in the ER lumen, activates the unfolded protein response (UPR), causes oxidative stress, and induces apoptosis in vitro and in vivo in mice. Strikingly, antioxidant treatment reduces UPR activation, oxidative stress, and apoptosis, and increases FVIII secretion in vitro and in vivo. The findings indicate that reactive oxygen species are a signal generated by misfolded protein in the ER that cause UPR activation and cell death. Genetic or chemical intervention to reduce reactive oxygen species improves protein folding and cell survival and may provide an avenue to treat and/or prevent diseases of protein misfolding.