The purpose of this study was to evaluate the reliability of volumetric magnetic resonance imaging (MRI) in breast cancer size assessment before, during and after neoadjuvant chemotherapy (NAC). Volumetric MRI measures performed on 15 patients with breast cancer were compared with volumes reckoned upon mean lesional diameters, using the same MRI data. Concordance correlation coefficient (CCC), Bland & Altman plots, RECIST evaluation and Cohen's Kappa were assessed, to evaluate the agreement between the two methods. CCC was computed before (0.9357), during (0.8053) and after (0.7499) NAC, in all examinations pooled together (0.8617), and on final tumor volume as a percentage of baseline volume (0.9224). In 2/15 (13.3%) cases RECIST assessment was different. Cohen's Kappa was 0.787 (CI(95%)=0.513-1.062). In summary, volumetric MRI is a reliable tool to assess breast cancer size before, during and after NAC. Further investigations are needed to understand whether improvements in surgical planning are feasible.