Indole-3-carbinol (I-3-C) is a naturally occurring constituent of many plant foods. Oral administration of I-3-C has been shown to manipulate estrogen metabolism in humans in a possibly beneficial manner. I-3-C increases the 2/16-hydroxyestrone ratio, a ratio found to be predictive of breast cancer risk in some prospective studies. Animal and in vitro studies have identified a number of other possibly beneficial effects of I-3-C and its metabolites, including inhibition of estrogen binding and modulation of oncogene expression. A chemopreventive effect of I-3-C has been demonstrated in a number of animal models. Some chemical carcinogenesis models have found a tumor promoting effect of I-3-C, however. Epidemiological studies support the hypothesis that high intakes of I-3-C may have broad chemopreventive effect. Preliminary human trials have demonstrated that I-3-C is well tolerated and has a sustained estrogen modifying effect. I-3-C is a good candidate for clinical trial in women at increased risk of developing breast cancer.