Epidemiologic evidence has linked diet to breast cancer, with the highest cancer rates observed in women who eat a high fat-low fiber diet. There is also substantial information, both clinical and experimental, that implicates estrogens in the etiology of breast cancer. A recent study from our laboratory has shown that diet influences levels of estrogens, and the main mechanism is metabolism of estrogens in the intestine. The intestinal microflora plays a key role in the enterohepatic circulation of estrogens by deconjugating bound estrogens that appear in the bile, thereby permitting the free hormones to be reabsorbed. By suppressing the microflora with antibiotic therapy, fecal estrogens increase and urinary estrogens decrease, changes indicating diminished intestinal reabsorption. A low fat-high fiber diet is associated with similar findings-high fecal estrogens and low urinary estrogens. It appears that the microflora plays a key role in the metabolism of female sex hormones.