PURPOSE: The purpose of this study was to determine whether magnetic resonance imaging (MRI) could assess the size of ductal carcinoma in situ (DCIS) more accurately compared to mammography and ultrasonography using the histopathological dimension of the surgical specimen as the reference measurement. METHODS: This was a retrospective review study using data from our institution database of breast cancer. Preoperative contrast-enhanced MRI, mammography and ultrasonography were performed to detect and assess the size of DCIS in 131 patients. The greatest dimensions of DCIS determined by the imaging modalities were compared with the histopathological dimensions of the surgical specimens. Intraclass coefficients were calculated to examine the agreement among the MRI, mammography and ultrasonography measurements. The Wilcoxon signed-rank test was used to evaluate the statistical significance of the differences in size among MRI, mammography or ultrasonography and histopathology findings. RESULTS: Of the 131 DCIS lesions, 126 (96.2%) were detected by MRI, 103 (78.6%) were detected by mammography, and 121 (92.4%) were detected by ultrasonography. The mean lesion size was 38.8 mm on histopathology, 36.0 mm on MRI, 28.8 mm on mammography, and 23.3 mm on ultrasonography, and there were no significant differences between sizes determined by histopathology and MRI, while there were significant differences between histopathology and the other modalities. The correlation coefficient between histopathological measurement and MRI was 0.837, versus 0.461 between histopathology and mammography and 0.284 between histopathology and ultrasonography. The lesion size was correctly estimated (±5 mm), under-estimated (<5 mm), or over-estimated (>5 mm), respectively, by MRI in 52.7%, 30.5%, and 16.8% of cases; by mammography in 32.0%, 51.2%, and 16.8% of cases, respectively; and by ultrasonography in 24.4%, 62.6%, and 13.0% of cases, respectively. CONCLUSION: In our study, MRI was more accurate for detection and assessment the size of DCIS compared to mammography and ultrasonography.