The cathepsin D concentration in 18 women with benign breast pathology has a cut-off value of 43 pmol/mg of protein. High values have been detected in two cases of chronic mastitis. These high values of cathepsin D were found in a study of 62 patients suffering from breast cancer and are independent of the hormone dependent state of the tumour. The cathepsin D concentration may have a prognostic function in breast cancer determination, as high concentrations are found in combination with other prognostic factors such as clinical stage, size of the tumour, state of the axillary lymph nodes and in the histological differentiation grade, where from a statistical point of view, the combination is important.