Fibromatosis is an uncommon breast lesion that can mimic breast carcinoma in its clinical presentation. We present a case in which excisional biopsy was necessary to establish a diagnosis of fibromatosis. Clinical, diagnostic imaging, and pathologic features are discussed. Magnetic resonance imaging (MRI) has emerged as a tool for further characterization of breast lesions and as a screening modality in high-risk patient populations. Ours marks the second case in which dynamic MRI has been correlated with histologically confirmed primary mammary fibromatosis. Unlike the previous report, MRI in this case mimics breast carcinoma in its morphologic and pharmacokinetic features of enhancement. Wide local excision with clear margins remains the treatment of choice. Current data on radiotherapy and pharmacologic therapy for mammary fibromatosis are reviewed.