Soybeans have been a major protein source for many centuries in Korea. Soybeans contain phytochemicals which are isoflavones, biochemically active component. Isoflavone is a kind of phytoestrogen, structurally and functionally similar to estrogen. It has been reported that the breast milk and blood of breast feeding mothers who consume soy products contain isoflavones. This study was conducted to investigate the effects of soy milk supplement on the isoflavones (daidzein, genistein) concentration of breast milk, plasma and urine from breast feeding woman. Seventeen healthy women who delivered at Kyung Hee Medical Center were recruited. For the first 2 weeks after delivery, seventeen women ingested 400 ml (isoflavone 43.2 mg) of soy milk on the given time starting from the day of giving birth. For the next 2 weeks, soy milk ingestion was withdrawn. Dietary intake and anthropometric data were checked and breast milk, blood, and 24 hr urine samples were collected on the day of giving birth, the 14th (the last day of the supplement phase) and 28th (the last day of the withdrawal phase) day, respectively. HPLC analysis was used to measure the concentration of isoflavones. Dietary intakes of the subjects were inadequate for the Korean RDA regardless of soy milk supplementation. Especially, intakes of vit A, calcium, and iron were very low. The Anthropometric data such as LBM, TBW, PIBW, BMI checked on the day of 14th decreased and maintained their levels by the 28th day. Daidzein concentration in breast milk was not affected by soy milk supplementation. However, genistein concentration decreased by the 28th day (14th day: 0.89 $pm$ 0.10 $mu$g/ml, 28th day : 0.48 $pm$ 0.07 $mu$g/ml) (p < 0.05). Plasma daidzein and genistein concentrations were not changed by the 14th day and decreased by the 28th day (14th day: 49.64 $pm$ 3.30 ng/ml, 26.72 $pm$ 2.90 ng/ml, 28th day: 38.30 $pm$ 4.40 ng/ml, 6.51 $pm$ 0.50 ng/ml, respectively) (p < 0.05). Twenty four hour urine concentrations of daidzein and genistein significantly increased by the 14th day and decreased by the 28th day (14th day: 5.80 :t 0.3 mg/d, 4.17 $pm$ 0.2 mg/d, 28th day: 6.72 $pm$ 0.4 mg/d, 5.09 $pm$ 0.5 mg/d, respectively) (p < 0.001). The rate of urinary recovery of daidzein was greater than that of genistein. The results of this study indicate that the supplement of dietary soy milk to the lactating women elevates the contents of isoflavone in the breast milk.