Breast cyst fluid (BCF) aspirated from 12 women with fibrocystic disease of the breast and sera obtained simultaneously were analyzed for bile acids. Analysis was performed by gas-liquid chromatography of the acetoxy methyl esters of the bile acids prepared after alkaline hydrolysis of the bile salts. An internal standard served to correct for methodological losses. Low levels of bile acids were found in serum samples, precluding overt hepatobiliary complications. Deoxycholic acid (17-160 mumol/L), chenodeoxycholic acid (18-305 mumol/L), and cholic acid (3-119 mumol/L) were detected in 11 of 12 samples of BCF. In 2 cases, chosen at random, the identities of the bile acids were verified by mass spectrometry. Lithocholic acid (9-23 mumol/L), a reported cocarcinogen, was detected in 6 of the 12 samples of BCF. This is the first report of the presence of lithocholic acid in BCF with confirmation by Mass spectrometry. There was no correlation between the levels of individual bile acids and those of potassium ion, Na+/K+, estriol-3-sulfate, or 16 alpha-hydroxyandrogen sulfates that had been quantified previously in these samples. There was borderline correlation between concentrations of total bile acids and K+ (P less than 0.06) and Na+/K+ (P less than 0.07). Yet to be elucidated are the mechanism of accumulation of bile acids in BCF and whether levels of particular bile acids in BCF may serve to identify that small subset of women with fibrocystic disease at risk for developing breast cancer.