The effect of dietary fiber on serum estrogen concentrations was studied in 62 premenopausal women. Their estimated daily dietary fiber intake was increased from an average of approximately 15 g to 30 g/d by supplementation with wheat, oat, or corn bran. This was done without any significant decrease in dietary fat consumption. After 2 mo on the high-fiber diets, the wheat-bran-supplemented group showed significant reductions in serum estrone (P less than 0.002) and estradiol (P less than 0.02) but no change in serum progesterone or sex-hormone-binding globulin concentrations. The serum estrogens were unaffected by dietary-fiber supplementation with oat or corn brans.