We report on two cases where breast magnetic resonance imaging examination changed clinical management. Breast magnetic resonance imaging is now recognised as an indispensable adjunctive examination to mammography and ultrasound. In each of the two cases described, breast magnetic resonance imaging revealed unsuspected, extensive, and mammographically and ultrasonologically occult, ductal carcinoma in situ. In each of these cases, planned breast conserving surgery was changed to mastectomy. The success of breast conservation treatment depends on removal of all tumour with clear margins at the time of surgery. Magnetic resonance imaging is now considered the most sensitive method for evaluating the extent of breast cancer. Breast magnetic resonance imaging has a very high sensitivity for invasive carcinoma (near 100%), and recent studies show its specificity in high-risk patients is between 93 and 99%. Magnetic resonance imaging may well be proven an important adjunctive examination in patients who have dense breasts or extensive fibrocystic change.