From laboratory studies presented, iodine appears to be a requisite for the normalcy of breast tissue in higher vertebrates. When lacking, the parenchyma in rodents and humans show atypia, dysplasia, and even neoplasia. Iodine-deficient breast tissues are also more susceptible to carcinogen action and promote lesions earlier and in greater profusion. Metabolically, iodine-deficient breasts show changes in RNA/DNA ratios, estrogen receptor proteins, and cytosol iodine levels. Clinically, radionuclide studies have shown that breast atypia and malignancy have increased radioactive iodine uptakes. Imaging of the breasts in high-risk women has localized breast tumors. The potential use of breast iodine determination to determine estrogen dependence of breast cancer has been considered and the role of iodide therapy discussed. In conclusion, iodine appears to be a compulsory element for the breast tissue growth and development. It presents great potential for its use in research directed toward the prevention, diagnosis, and treatment of breast cancer.