Internal mammary lymph nodes constitute a major lymphatic chain draining the breast and a route of spread for breast cancer metastases. Both physiologic and metastatic internal mammary lymph nodes enhance on breast magnetic resonance imaging, and the clinical significance of their prevalence, size, and morphology when visualized in a patient with breast cancer remains unknown. We studied the characteristics of internal mammary lymph nodes visualized on breast MRI studies before and after neo-adjuvant therapy in twenty-three patients with newly diagnosed breast cancer. A measured decrease in internal mammary lymph node size on post-neo-adjuvant therapy MRI indicated metastatic involvement. Determining suspicious features of internal mammary nodes on initial diagnostic MRI can aid radiologists in reporting probable IMLN metastases and may alter the course of care for patients with breast cancer. This study concludes that metastatic internal mammary lymph nodes should be considered when more than two ipsilateral internal mammary lymph nodes measuring 6 mm or greater are seen on diagnostic MRI in a patient with newly diagnosed breast cancer.