Animal studies have shown that soy isoflavones have an effect in preventing estrogen-related bone loss, but few data are available in humans, especially in the Asian populations. This double-blind, placebo-controlled, randomized trial examines the effects of soy isoflavones on bone loss in postmenopausal Chinese women, aged 48-62 yr. Two hundred and three eligible subjects were randomly assigned to three treatment groups with daily doses of placebo (1 g starch; n = 67), mid-dose (0.5 g starch, 0.5 g soy extracts, and approximately 40 mg isoflavones; n = 68), and high dose (1.0 g soy extracts and approximately 80 mg isoflavones; n = 68). All were given 12.5 mmol (500 mg) calcium and 125 IU vitamin D(3). Bone mineral density (BMD) and bone mineral content (BMC) of the whole body, spine, and hip were measured using dual energy x-ray absorptiometry at baseline and 1 yr post treatment. Both univariate and multivariate analyses showed that women in the high dose group had mild, but statistically significantly, higher favorable change rate in BMC at the total hip and trochanter (P < 0.05) compared with the placebo and mid-dose groups, even after further adjustments for the potential confounding factors. Further stratified analyses revealed that the positive effects of soy isoflavone supplementation were observed only among women with lower initial baseline BMC (median or less). In conclusion, soy isoflavones have a mild, but significant, independent effect on the maintenance of hip BMC in postmenopausal women with low initial bone mass.