Iron is an essential mineral for normal cellular physiology, but an excess can result in cell injury. Iron in low-molecular-weight forms may play a catalytic role in the initiation of free radical reactions. The resulting oxyradicals have the potential to damage cellular lipids, nucleic acids, proteins, and carbohydrates; the result is wide-ranging impairment in cellular function and integrity. The rate of free radical production must overwhelm the cytoprotective defenses of cells before injury occurs. There is substantial evidence that iron overload in experimental animals can result in oxidative damage to lipids in vivo, once the concentration of iron exceeds a threshold level. In the liver, this lipid peroxidation is associated with impairment of membrane-dependent functions of mitochondria and lysosomes. Iron overload impairs hepatic mitochondrial respiration primarily through a decrease in cytochrome C oxidase activity, and hepatocellular calcium homeostasis may be compromised through damage to mitochondrial and microsomal calcium sequestration. DNA has also been reported to be a target of iron-induced damage, and this may have consequences in regard to malignant transformation. Mitochondrial respiratory enzymes and plasma membrane enzymes such as sodium-potassium-adenosine triphosphatase (Na(+) + K(+)-ATPase) may be key targets of damage by non-transferrin-bound iron in cardiac myocytes. Levels of some antioxidants are decreased during iron overload, a finding suggestive of ongoing oxidative stress. Reduced cellular levels of ATP, lysosomal fragility, impaired cellular calcium homeostasis, and damage to DNA all may contribute to cellular injury in iron overload. Evidence is accumulating that free-radical production is increased in patients with iron overload. Iron-loaded patients have elevated plasma levels of thiobarbituric acid reactants and increased hepatic levels of aldehyde-protein adducts, indicating lipid peroxidation. Hepatic DNA of iron-loaded patients shows evidence of damage, including mutations of the tumor suppressor gene p53. Although phlebotomy therapy is effective in removing excess iron in hereditary hemochromatosis, chelation therapy is required in the treatment of many patients who have combined secondary and transfusional iron overload due to disorders in erythropoiesis. In patients with beta-thalassemia who undergo regular transfusions, deferoxamine treatment has been shown to be effective in preventing iron-induced tissue injury and in prolonging life expectancy. The use of the oral chelator deferiprone remains controversial, and work is continuing on the development of new orally effective iron chelators.