The aim of this study was to evaluate calreticulin expression in infiltrating ductal breast carcinomas (IDCAs), as well as its relationships with clinicopathological parameters of the disease. Using a two-dimensional gel electrophoresis/matrix-assisted laser desorption ionization time of flight mass spectrometry investigation coupled to an immunohistochemical approach, we have assessed the expression of calreticulin in IDCAs, as well as in other types of breast tumors. The humoral immune response against calreticulin was estimated using a serological proteomics-based strategy. Proteomic analyses revealed an increased expression of calreticulin in IDCA tumors. Using immunohistochemistry, overexpression of calreticulin was confirmed in 51 additional tumor specimens. Statistical analyses revealed, however, no significant correlations between calreticulin expression and clinicopathological parameters of the disease including tumor stage, patient age, SBR grade, and lymph node metastasis occurrence. A significant association was found, however, with estrogen receptor status. This study demonstrates the upregulation of calreticulin in IDCA tissues which may highlight its involvement in breast cancer development. Our findings also support a link between calreticulin expression and estrogen transduction pathways. Our results do not, however, support the involvement of calreticulin in the development of a humoral immune response in IDCAs.