Iron participates in a range of reactions that are necessary for cell viability and cell proliferation. Iron is an essential component in DNA synthesis and in respiratory and oxidative metabolism. These functions relate to the properties of unremitting proliferation and a more anaerobic metabolism, that may contribute to a selective advantage of neoplastic cells over nonneoplastic cells. Clinical correlations have been made linking cellular iron content to the development of cancer in humans. The clinical entities include disease states in which there is abnormal accumulation of iron as part of the disease process, and cases in which neoplasms have developed as a result of administered iron preparations. The molecular mechanisms regulating cellular iron incorporation and the iron-dependent formation of reactive oxygen intermediates that can cause cell injury have been recently elucidated and provide a basis for better understanding the relationship of these processes to neoplastic development.