Background and Aim: in landspitali university hospital, magnetic resonance imaging is used non-selectively in addition to mammogram and ultrasound in the preoperative assessment of breast cancer patients. The aim of this study was to assess invasive tumor size on imaging, compare with pathological size and evaluate the impact of magnetic resonance imaging on the type of surgery performed.Material and Methods: all women with invasive breast cancer, diagnosed in iceland, between 2007 and 2009 were reviewed retrospectively. in all, 438 of 641 (68%) patients diagnosed had preoperative magnetic resonance imaging. Twelve patients treated with neoadjuvant chemotherapy were excluded and 65 patients with multifocal or contralateral disease were assessed separately.Results: Correlations between microscopic and radiologic tumor sizes were relatively weak. all imaging methods were inaccurate especially for large tumors, resulting in an overall underestimation of tumor size for these tumors. magnetic resonance imaging under-and overestimated pathological tumor size by more than 10 mm in 16/348 (4.6%) and 26/348 patients (7.5%), respectively. in 19 patients (73%), overestimation of size was seen exclusively on magnetic resonance imaging. for tumors under-or overestimated by magnetic resonance imaging, the mastectomy rates were 56% and 65%, respectively, compared to an overall mastectomy rate of 43%. of 51 patients diagnosed with multifocal disease on pathology, 19 (37%) were diagnosed by mammogram or ultrasound and 40 (78%) by magnetic resonance imaging resulting in a total detection rate of 84% (43 patients).