['The aim of our study was to establish which clinical, radiologic and pathologic factors could predict the risk of under- and overestimation of the breast ductal carcinoma in situ (DCIS) size when preoperatively measuring the maximum mammographic extent of microcalcifications (MEM).', 'We made a retrospective review of patients with a DCIS treated in our Breast Unit between May 2005 and May 2012. Clinical, pathologic and radiologic data were evaluated as possible predictive factors for over- or underestimation of DCIS size when measuring MEM.', 'We obtained precise measurements of MEM in 82 patients (84 DCIS lesions). Maximum MEM measurement correctly estimated maximum pathology size in 57 lesions (68.7\xa0%). Patients with a correctly estimated DCIS, with an underestimated DCIS and with an overestimated DCIS significantly differed in DCIS ER expression (p\xa0=\xa00.022) and in maximum MEM measurement (p\xa0=\xa00.000). Constructing two ROC curves, we found that a maximum MEM measurement ≥25\xa0mm and ER expression ≥90\xa0% were both discrimination points for overestimation and ER\xa0≤\xa045\xa0% was a discrimination point for underestimation. Using these cutoff points, we defined four groups of patients with different risks of over- and underestimation.'] Do mammographic extent of microcalcifications and oestrogen receptor expression affect preoperative breast carcinoma in situ size estimation?