For the purpose of investigating a possible correlation between the genesis of breast cancer and the levels of serum thyroid hormones or the estrogen status, which is one of the potential risk factors for breast cancer in Japanese women, we measured the percentage of free estradiol (E2) and the amounts of sex hormone-binding globulin (SHBG) and thyroid hormones in serum samples from Japanese patients with breast cancer (N = 39) and normal controls (N = 36). The patients were found to have significantly higher free E2 and significantly lower SHBG than controls. Moreover, the serum levels of free triiodothyronine (FT3) and free thyroxine (FT4) were lower in the patients than in controls, while the serum levels of TSH and TBG in the patients were not significantly different from those in controls. The percentage of free E2 in serum was not significantly correlated with the level of any one of FT3, FT4, TSH, and TBG either in the patients or in controls regardless of menstrual status. These results suggest the possibility that the reduction in the serum FT3 and FT4 levels, which is independent of changes in the serum level of free E2, may be one of the risk factors for breast cancer in Japanese women.