The levels of cortisol, progesterone, testosterone, dihydrotestosterone, androsterone, androsterone sulfate, dehydroisoandrosterone, dehydroisoandrosterone sulfate, estradiol, estrone, estradiol sulfate and estrone sulfate were measured in human breast fluid. The levels of the four 17-ketosteroids and the 2 estrogen sulfates were markedly elevated over the plasma level, while that of the other compounds was the same or only slightly higher than the plasma levels of the same compounds. [The relationship between the levels of these hormones and the risk of breast cancer is discussed.].