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 cm2 for placebo vs. -11.86 ± 31.48 cm2 for soy; subcutaneous abdominal fat: +22.91 ± 28.58 cm2 for placebo vs. -14.73 ± 22.26 cm2 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. © 2007 American Society for Reproductive Medicine.