Breast tissue obtained from 18 of 246 women (7.3%) having localization biopsy for microcalcification contained crystals of Weddellite (calcium oxalate dihydrate). This was characterized by microscopic infra-red spectroscopy in three cases. Weddellite calcification was associated with benign lesions in 16 cases, but incidental atypical lobular hyperplasia and lobular carcinoma in situ were present, each in one case. Pathologists should be aware of this common non-haematoxyphilic, birefringent form of breast micro-calcification. Its presence has implications for the examination of biopsy specimens resulting from breast cancer screening using mammography.