Cathespin D (Cath D) is a proteolytic enzyme secreted by human breast cancer cells with a growth promoting activity in vitro. In the present study, we measured Cath D and Epidermal Growth Factor/alpha Transforming Growth Factor (EGF/alpha-TGF) concentrations in the breast cyst fluid (BCF) of 43 patients with gross cystic disease of the breast. Both Cath D (median 2.45 pmoles mg-1 protein; range 0-4.84 vs 0.98 pmoles mg-1 protein; range 0-3.11) and EGF/alpha-TGF (28.71 ng mg-1 protein; range 7.05-50.63 vs 10.83 ng mg-1 protein; range 0.06-30.55) levels were higher in BCF of apocrine than flattened cysts (P less than 0.0005 and P less than 0.01, respectively). Premenopausal patients showed higher concentrations of Cath D (P less than 0.05) and EGF/alpha-TGF (P less than 0.05) than postmenopausal patients. A positive correlation was obtained between intracystic concentrations of Cath D and EGF/alpha-TGF (P less than 0.00001). The higher levels of Cath-D and EGF/alpha-TGF found in apocrine cysts could provide an explanation for the increased risk of subsequent breast cancer in women with this type of cyst.