Cathepsin D is a lysosomal hydrolase involved in intra- and extracellular proteolysis. This enzyme is aberrantly produced and processed in malignancy, and most notably is over-secreted into the tumor cell microenvironment. This hyper-secretion may lead to excessive degradation of the extracellular matrix, and contribute to tumor progression and metastases. These phenomena have been established in vitro, and there is evidence that Cathepsin D is similarly dysregulated in human breast cancer patients. Because breast cancer lacks an effective screening or surveillance biomarker, here we address the hypothesis that serum Cathepsin D activity may be useful to assess the presence or progression of breast cancer in females. While representative histologic sections from various disease-specific cohorts confirm previous findings that increased Cathepsin D production and secretion correlate with tumor progression, we report no difference in serum Cathepsin D activity between patients who are disease free, patients with pre-invasive or limited invasive disease, and patients with metastatic disease. Furthermore, in patients with known metastatic disease, there were no clinical variables associated with significantly different serum Cathepsin D activity. However, the immunohistochemical localization of Cathepsin D expression in histopathologic sections from breast cancer patients correlates with disease progression. Based on the serum results, and in contradistinction to Cathepsin D localization in breast cancer tissues, our findings support using Cathepsin D as a reliable histopathology biomarker for disease progression, but not for serum screening.