OBJECTIVE: To determine whether a supplement of soy protein improves body composition, body fat distribution, and glucose and insulin metabolism in postmenopausal women without diabetes compared with an isocaloric casein placebo. DESIGN: Randomized, double-blind, placebo-controlled 3-month trial. SETTING: Clinical Research Center. PATIENT(S): Fifteen postmenopausal women. INTERVENTION(S): Computed tomographic scans at L4/L5, dual energy x-ray absorptiometry, hyperglycemic clamps. MAIN OUTCOME MEASURE(S): Total fat, total abdominal fat, visceral fat, subcutaneous abdominal fat, and insulin secretion. RESULT(S): Weight by dual energy x-ray absorptiometry did not change between groups (+1.38 +/- 2.02 kg for placebo vs. +0.756 +/- 1.32 kg for soy, mean +/- SD). Total and subcutaneous abdominal fat increased more in the placebo group than in the soy group (for differences between groups in total abdominal fat: +38.62 +/- 22.84 cm(2) for placebo vs. -11.86 +/- 31.48 cm(2) for soy; subcutaneous abdominal fat: +22.91 +/- 28.58 cm(2) for placebo vs. -14.73 +/- 22.26 cm(2) for soy). Insulin secretion, visceral fat, total body fat, and lean mass did not differ between groups. Isoflavone levels increased more in the soy group. CONCLUSION(S): A daily supplement of soy protein prevents the increase in subcutaneous and total abdominal fat observed with an isocaloric casein placebo in postmenopausal women.