Several reports indicate that patients with macrocysts have a two- to fourfold higher risk of developing cancer. The fluid filling the cysts (breast cyst fluid, BCF) contains unusual amounts of biologically active substances, including hormones and metabolites. The accumulation of steroid conjugates, such as androgen and estrogen sulfates, deserves interest. Measuring BCF cations (K+, Na+) permits classification of cysts into two major subsets (type I and type II), conceivably associated with a different degree and with a turnover of apocrine cells in the lining epithelium. Type I (high K+/Na+ ratio) and type II (low K+/Na+ ratio) cysts display different patterns of steroid analytes and steroid-binding proteins. There are many gaps in our understanding of the relationship between local steroids and hypersecretion of fluid in the terminal duct lobular units with eventual appearance of cysts. Accumulating biochemical and epidemiological data, however, point to recurrent, multiple type I cysts as a marker of endocrine risk, i.e., of a whole-organ promoting status toward proliferative premalignant lesions.