Radiographic mammary calcifications occur in 30-50% of breast cancers and constitute one of the most important diagnostic markers of both benign and malignant lesions of the breast. The presence of oxalate-type microcalcification appears to be a reliable criterion in favor of the benign nature of the lesion or, at most, of a lobular carcinoma in situ. In contrast, calcium hydroxyapatite (HA) crystals are associated with both benign and malignant breast tumors. Although the diagnostic value of microcalcifications in breast cancer is of great importance, the genesis of these calcifications is unclear. Despite numerous histological ultrastructure studies of HA deposits in breast carcinomas, to date there have been limited investigations of the potential role of these crystals in breast cancer. We review the literature examining the biological effects of HA crystals in breast cancer cell lines, specifically the mechanism of HA-induced mitogenesis and upregulation of gene expression.