Estrogens have a variety of beneficial effects in vivo including protection against osteoporosis, coronary heart disease, Alzheimer's disease, and stroke. Similarly, antiestrogens have been shown to be effective both in treating breast cancer as well as preventing this disease. Despite the health benefits of these drugs adverse side effects have been reported including increased risk for developing certain hormone dependent cancers. Although an estrogenic mechanism likely contributes to mechanism of estrogen/antiestrogen carcinogenesis there is substantial evidence to suggest that metabolism to reactive intermediates is also involved. Both estrogens and antiestrogens can be metabolized to phenoxyl radicals, o-quinones, and semiquinone radicals, all of which could cause damage in cells either through alkylation or oxidation of cellular macromolecules including DNA. In contrast, there are several reports that estrogens and antiestrogens can act as antioxidants which could protect cells against free radical mediated damage and contribute to the beneficial effects of these compounds discussed above. The focus of this review is the role of quinoids, quinoid radicals, and phenoxyl radicals in the biological effects of estrogens and antiestrogens.