To evaluate chronic effect of polychlorinated biphenyls (PCB) and polychlorinated dibenzofurans (PCDF) on sex hormones and prolactin, serum levels of estradiol, progesterone and prolactin were studied in 71 female patients with Yusho and 23 controls in 2006. A negative correlation was found between blood PCB concentrations and estradiol levels. There were no significant correlations between blood PCDF concentrations and estradiol levels, progesterone levels or prolactin levels. However, the mean serum estradiol level was significantly lower in 29 patients with high PCDF concentration (higher than 200 pg/g lipid in blood) than in 30 patients with low PCDF concentration (lower than 200 pg/g lipid in blood). We conclude that abnormality of estradiol levels may be associated with blood PCB concentration and blood PCDF concentration in patients with Yusho.