The concentration of potassium (K+) and sodium (Na+) was measured in breast cyst fluid (BCF) from 611 cysts greater than 3 ml aspirated in 520 women with gross cystic disease of the breast. These women were enrolled, from 1983 on, in a cohort study aimed at assessing the relationship between cyst type, as defined by the K+/Na+ ratio in BCF, and the risk of breast cancer. The inverse relationship between K+ and Na+ and the bimodal distribution of the K+/Na+ ratio in BCF were confirmed. Type I cysts were defined as cysts with a K+/Na+ greater than 1.5 in BCF. Among women with type I cysts, a higher proportion of women with one or no births, of women with a history of apocrine cysts, of current smokers, and of women who do not drink coffee was found, as compared to women with other types of cysts. The risk of cyst relapse was significantly higher among women with type I cysts than among women with other types of cysts and among women with multiple cysts at presentation. These findings indicate that type I BCF is a marker of "active" gross cystic disease of the breast and suggest that it may be associated with increased breast cancer risk.