620 Background: We presented 3D-MRI will be the useful breast imaging tool in diagnosing intraductal spread and tumor size in breast cancer patients. (ASCO 2008).To do breast conserving surgery, careful management was needed in cases with widely ductal spreading and multifocal lesion. This study was to evaluate factors influencing false negative diagnosis and multifocal types of 3D-MRI, retrospectively. Methods: From July 2000, pre-operative 3D-MRI was underwent 488 cases and images were obtained 479 cases (98%). These patients could be divided into three groups according to 3D-MRI images: pattern 1) localized type (273 cases), 2) widely enhanced type (106 cases) and 3) multi-focal type (100 cases). From April 2005, resected specimen was serially step cut and total specimens were microscopically evaluated. Tumor size in 3D-MRI was 1cm over smaller than resected specimens were defined false negative cases. Investigated factors influencing 3D-MRI were age, patterns of 3D-MRI images, presence of pre-operative chemotherapy, histological type, histological grade, microscopical invasive tumor size (mm) and presence of ductal spreading. Results: The rate of histological confirmed intraductal spread were seen in 58% (158/273cases), 89% (94/106 cases) of patients with 3D-MRI pattern 1) and 2), respectively. Tumor size was correlated between MRI and specimens, significantly (n = 162, r = 06706, p < 0.001). Factors influencing false negative diagnosis of 3D-MRI were presence of pre-operative chemotherapy (p < 0.05), presence of ductal spreading (p < 0.001) and patterns of 3D-MRI images (p < 0.05). In the cases with multi-focal 3D- images, there were only 11(11/41, 26%) cases with true multifocal lesions by total specimens examination. Selection for breast conserving surgery and margin width were based by 3D-MRI images. The proportion of breast-conserved surgery were obtained 90% (437/488 cases) and local failure seen only 6 cases (local failure rate 1.4%) in 3D-MRI guided surgery. (median follow up 52 months). Conclusions: These results suggest that MRI will be the useful breast imaging tool, but caution will be needed in cases with pre-operative systemic therapy, cases expected intraductal spread by imaging tools and cases with multi-focal 3D- images. No significant financial relationships to disclose.