The risk of developing breast cancer is higher in women presenting gross cystic disease (cysts > 3 mm in diameter) of the breast with intracystic K+/Na+ > 3 as compared with K+/Na+ < 3. The present study reports the levels of tumour necrosis factor-alpha (TNF-alpha), interleukin-1 (IL-1), and interleukin-6 (IL-6) in the breast cyst fluid of women with gross cystic disease and analyses the relationship between the intracystic concentration of these cytokines, sex steroid hormones, and the K+/Na+ ratio. The concentration of these cytokines, estradiol, testosterone, dehydroepiandrosterone sulfate (DHEA-S), and 17-OH-progesterone were determined in the breast cyst fluid of 54 women with gross cystic disease. No significant differences were found in the cystic levels of IL-1 between cysts with intracystic K+/Na+ < 3 and > 3. However, in cysts with intracystic K+/Na+ > 3 we found a lower concentration of IL-6 and TNF-alpha than in those with intracystic K+/Na+ < 3. Stepwise multiple linear regression analysis demonstrated that the concentration of IL-6 in breast cyst fluid was predicted statistically by a negative regression coefficient for the concentration of estradiol and DHEA-S, and by a positive regression coefficient for the concentration of TNF-alpha. The concentration of TNF-alpha in breast cyst fluid was predicted statistically by a positive regression coefficient for the concentration of IL-6, and by a negative regression coefficient for the concentration of estradiol. No candidate variable was included in the model to predict concentrations of IL-1 in breast cyst fluid. Our results indicate that IL-6 and TNF-alpha could have a local 'protector' role in gross cystic disease, and that they could be used as a marker to identify cyst type.