A 36-year-old woman presented with a 10 mm diameter mass in the right breast. Since the mass persisted for 3 months after detection and mammary carcinoma was suspected based on ultrasonographic findings, the mass was resected. Histologically, the mass demonstrated proliferative margins and consisted of spindle cells with bland cytologic features and abundant collagen. Immunohistochemically, the spindle cells were positive for vimentin and smooth muscle actin, and were negative for cytokeratins and desmin. Furthermore, the cells showed MIB-1 immunoreactivity with a MIB-1 labeling index of 4.1. Based on these findings, was diagnosed fibromatosis. Breast fibromatosis is rare and is usually misdiagnosed as breast carcinoma preoperatively. To date, only 10 cases of breast fibromatosis have been reported in Japan. Among the reported cases in Japan, our patient presented with the smallest mass, and ultrasonographic findings in this case were the same as those of other cases. Our experience and a review of the literature indicated that differentiation of fibromatosis from carcinoma is very difficult by ultrasonographic examination. In our case, despite involvement of the surgical margins, there was no recurrence. This may be attributed to the small size of the mass and focal exposure.