Radioimmunoassayable levels of total and free 3,5,3'-triodothyronine (T3) and thyroxine (T4), thyroid stimulating hormone (TSH), thyroxine binding globulin (TBG) and prolactin (Prl) were measured in a series of samples of breast cyst fluid (BCF) aspirated from 73 pre-menopausal women with gross cystic disease of the breast and in coincidental blood samples. In BCF free T3 and free T4 were consistently higher than in plasma (P less than 0.001) as was also total T3 in the majority of cases (P less than 0.05). Total T4, TBG and Prl were much lower than in plasma (P less than 0.001). TSH was roughly as in plasma. No correlation was found between plasma and BCF values. The results indicate that thyroid hormones may be accumulated in their effective form in the cyst fluid. A highly significant inverse correlation was found between intracystic free T3 and log (Prl). Our data, considered in relation to previous experimental evidence, suggest the possibility that thyroid hormones act at the breast level in concert with Prl. Accumulation of thyroid hormones in BCF could play a role in the development or maintenance of gross cystic disease of the breast or be a mere consequence of the cystic changes.