Gross cystic breast disease may be categorised into 2 groups according to the intracystic cation ratios (Na+/K+ less than 3 and Na+/K+ greater than 3). It has been suggested that women in the low electrolyte ratio group may be at higher risk of developing breast cancer. Epidermal growth factor concentrations in breast cyst fluid were significantly higher in the low than in the high electrolyte ratio group but there was no significant difference in oestradiol concentrations between the two groups of breast cysts. A negative correlation was obtained between intracystic concentrations of epidermal growth factor and Na+/K+ (rs = -0.617, p less than 0.001). No evidence was found to support the hypothesis that concentrations of epidermal growth factor may be modulated by oestradiol. The higher concentrations of epidermal growth factor in the low electrolyte ratio group may provide an explanation for the higher risk of breast cancer observed in this group.