Deposition of basement membrane extracellular matrix is influenced by adjacent tumor cells, and in some cases, the pattern of type IV collagen deposit is characteristic in malignant tumors. In this report, we analyzed the difference in type IV collagen deposition patterns between benign and malignant phyllodes tumors (PTs) of the breast. Of the 15 cases of PTs, 8 cases were benign PTs and 7 cases were malignant PTs. Three cases of other primary sarcomas of the breast (stromal sarcoma, angiosarcoma and osteosarcoma) and 2 cases of fibroadenomas were studied for comparison. The malignant PTs were distinguished from benign ones by increased mitotic figures, cellular atypism, and a higher proliferation index of stromal cells. Immunohistochemical staining against type IV collagen in malignant PTs revealed extensive to moderate deposition of type IV collagen around the small blood vessels in duplicate or multilayering pattern, while benign PTs showed minimal deposition in a single linear pattern. All of the three cases of other sarcomas revealed multilayering or meshwork pattern of type IV collagen around the blood vessels. The deposition of type IV collagen around the blood vessels may reflect the malignant behavior of the stromal tumors of the breast.