Gross cystic breast disease is a benign lesion occurring in 7% of adult women. Apocrine changes of epithelium lining the breast cysts cause a higher risk of developing breast cancer. According to the possible role of bile acids in the pathogenesis of cancer, we analysed breast cyst fluids aspirated from 96 women for distribution of conjugated bile acid concentrations in the two subsets of breast cysts. Bile acid levels were correlated to K+ concentrations (P < 0.0001) and mean value was higher in Na/K < 3 metabolically active apocrine cyst as compared with Na/K > 3 flattened cyst (P < 0.001). Because bile acids could play an important role in the pathogenesis and growth of breast cancer, the significantly higher intracystic concentrations of these carcinogen compounds in apocrine Type I cysts might provide a further biological explanation as to why women with apocrine changes may be at higher breast cancer risk and could be useful for the biochemical knowledge occurring in the different functional stages of the gross breast cysts.