The short-term effects of a low-fat (less than 10% of cal) high-fiber (35-45 g.1000 kcal-1.day-1) diet on estradiol levels were studied in 13 postmenopausal women under residential conditions with cafeteria food service. During this diet study (mean +/- SE duration 22 +/- 4.6 days), body weight declined from 84.9 +/- 6.4 to 82.3 +/- 6.0 kg (P less than 0.001). Serum cholesterol and low-density and high-density lipoprotein cholesterol also fell significantly, whereas triglycerides remained constant. Serum estradiol fell from 18.1 +/- 3.6 to 9.4 +/- 2.4 pg/ml (P less than 0.05). This study demonstrates that serum estradiol levels are reduced in normal to over-weight postmenopausal women given free access to a low-fat high-fiber diet. Most of these women were in negative caloric balance during the study, suggesting that short-term changes in caloric balance may affect estrogen synthesis in postmenopausal women prior to a large decrease in fat mass.