The effect of diet on serum estrogen levels was investigated in 17 healthy premenopausal women consuming defined diets prepared in a metabolic unit. During an initial 4-wk control period all women consumed a typical Western diet (40% of total calories from fat, 400 mg cholesterol/d, 12 g dietary fiber/d, and a ratio of polyunsaturated to saturated fatty acids [P:S] of 0.5). After this control period they were switched to a low-fat, high-fiber diet for 8-10 wk, which consisted of 25% of calories from fat, P:S of 1.0, cholesterol of 200 mg cholesterol/d, and 40 g dietary fiber/d. Compared with the control period 16 of 17 women had lower serum estrone sulfate levels on the low-fat, high-fiber diet. There was an average decrease of 36% with mean levels decreasing from 2.11 +/- 0.25 nmol/L (means +/- SEM) on the control diet to 1.29 +/- 0.19 nmol/L on the experimental diet (p less than 0.001). We conclude that a low-fat, high-fiber diet can significantly reduce serum estrone sulfate levels.