Purpose: Accurate preoperative assessment of breast cancer is important to determine the extent of disease and the plan for surgery. The purpose of this study was to evaluate the efficacy of preoperative breast magnetic resonance imaging (MRI) in breast cancer patients. Methods: Between January 2001 and October 2007, 457 consecutive patients who had undergone surgical treatment for breast cancer were retrospectively studied. We compared 303 patients from the non-preoperative MRI group (group A) to 154 patients from the preoperative MRI group (group B). The impact of preoperative MRI was evaluated for each patient with regard to changes in therapeutic intervention. Results: MRI alone revealed 17 new lesions. The results of the MRI led to a change in 9.1% of the planned surgical procedures. Tumor size was more accurately defined in patients undergoing MRI than in those undergoing ultrasound imaging. Conclusion: Breast MRI could be recommended as a preoperative diagnostic procedure in patients allocated to receive breast conservation surgery, because MRI may reveal unsuspected multifocal or multicentric tumors or carcinoma infiltrations and may result in changes in therapy.