Fibrous lesions of the breast are challenging for a number of reasons. They occur infrequently and there is significant clinical, radiologic, and histologic overlap between reactive and neoplastic lesions, as well as with some nonfibrous lesions. Pure fibrous lesions of the breast encompass reactive conditions, such as scars and nodular fasciitis, and neoplastic entities, such as the distinct benign mammary myofibroblastoma, locally aggressive fibromatosis, and rare true sarcomas. In this review, we focus on nodular fasciitis, mammary myofibroblastoma, and primary mammary fibromatosis. We also briefly discuss spindle cell metaplastic carcinoma because it represents the most important differential diagnosis through its ability to closely simulate nodular fasciitis and fibromatosis. Caution should be exercised in the interpretation of fibrous lesions of the breast, especially if the tissue sample is limited. Int J Surg Pathol 8(2):99-108, 2000