The steady-state formation of prooxidants in cells and organs is balanced by a similar rate of their consumption by antioxidants that are enzymatic and/or nonenzymatic. "Oxidative stress" results from imbalance in this prooxidant-antioxidant equilibrium in favor of the prooxidants. A number of diseases are associated with oxidative stress, being the basis of a potential antioxidant therapy. However, current evidence in clinical research does not show unequivocal distinction between causal or associative relationships of proxidants to the disease process.