Milk has more beneficial effects on bone health compared to other food sources. Recent in vitro and in vivo studies showed that milk whey protein, especially its basic protein fraction, contains several components capable of both promoting bone formation and inhibiting bone resorption. However, the effects of milk basic protein (MBP) on bone metabolism of humans are not known. The object of this study was to examine the effects of MBP on bone metabolism of healthy adult women. Thirty-three normal healthy women were randomly assigned to treatment with either placebo or MBP (40 mg per day) for six months. The bone mineral density (BMD) of the left calcaneus of each subject was measured at the beginning of the study and after six months of treatment, by dual-energy x-ray absorptiometry. Serum and urine indices of bone metabolism were measured at the base line, three-month intervals, and the end of the study. Daily intake of nutrients was monitored by a three-day food record made at three and six months. The mean (+/- SD) rate of left calcaneus BMD gain of women in the MBP group (3.42 +/- 2.05%) was significantly higher than that of women in the placebo group (2.01 +/- 1.75%, P = 0.042). As compared with the placebo group, urinary cross-linked N-teleopeptides of type-I collagen/creatinine and deoxypyridinoline/creatinine were significantly decreased in the MBP group (p < 0.05), while no significant differences between the two groups were observed in serum osteocalcin and bone-specific alkaline phosphatase concentrations. A daily MBP supplementation of 40 mg in healthy adult women can significantly increase their BMD independent of dietary intake of minerals and vitamins. This increase in BMD might be primarily mediated through inhibition of osteoclast-mediated bone resorption by the MBP supplementation.