The relationship between cathepsin D and other pathological or biological prognostic parameters has not yet been defined through systematic studies in breast cancer. The aim of the present investigation was to define the relationship between cathepsin D and nodal status, tumour size, steroid receptors and tumour grade in a wide patient series. Cytosol cathepsin D was assayed with an immunoradiometric assay in tumour samples from 1752 patients. A statistically significant, but not biologically meaningful association was found between cathepsin D and both tumour size and grade. Cathepsin D was significantly higher in node-positive than in node-negative tumours. However, cathepsin D is not of great use in order to predict the risk of axillary metastases in individual patients, due to overlapping of cathepsin D values between node-positive and node-negative cases. A significant, direct association was found between cathepsin D and both oestrogen receptor and progesterone receptor cytosol levels. Nevertheless, preliminary data indicate that cathepsin D and steroid receptors provide independent prognostic information.