Cathepsin D is an acidic lysosomal protease expressed in all cells. Some studies have shown correlations between high levels of tissue cathepsin D and poor prognosis. This paper deals with 158 cases of breast cancer in which tissue concentrations in cathepsin D, age, estrogen and progesterone receptor content, and pathological characteristics of the tumor were investigated. Tumors were considered to be cathepsin D+ when a concentration > 40 pmol/mg protein (median value in our samples) was determined. The expression of cathepsin D appears to be related to grading (p = 0.04) and lymph node status (p = 0.05). We found no significant associations among cathepsin D levels, patient age, steroid receptors and histological type. Moreover, the levels of cathepsin D have been evaluated in 9 samples of recurring or metastatic neoplasia and 11 cases of benign breast lesions. We conclude that cathepsin D may be a useful prognostic predictor in breast cancer. Further investigations are required to improve and extend the applications of this assay.