Although cystic lesions of the breast are not precancerous per se, statistical studies have indicated that this condition predisposes a 2- to 4-fold greater risk for breast cancer. Seeking a hormonal etiology to this correlation, investigators have analyzed breast cyst fluid ( BCF ) for steroids and have compared the levels to those in the blood. The 17-ketosteroids-androsterone, dehydroisoandrosterone and their sulfates are elevated in BCF . The same is true for estrone sulfate and estradiol-3-sulfate. We have found the most dramatic differences with estriol-3-sulfate (E3-3S), the concentration of which ranged from 187-6134 pg/ml in over 40 specimens analyzed, whereas in 12 serum specimens from normal women, E3-3S was barely detectable. The origin of E3-3S is not known. [3H]E3-3S is not concentrated in BCF following its injection into an arm vein. The blood half-life of [3H]E3-3S is much lower than that of estrone sulfate. Samples of breast nipple aspirates from normal women were also analyzed for E3-3S. None could be detected. The best explanation of the data accumulated thus far is that E3-3S is synthesized at the epithelial lining of the cyst and released into the BCF , from which its efflux is inefficient.