Pseudoangiomatous hyperplasia of mammary stroma (PHMS) is a benign proliferation of keloid-like fibrosis, containing slit-like pseudovascular spaces. Its main importance is its distinction from angiosarcoma; however, the clinicopathologic spectrum of PHMS remains incompletely described. We report two new cases and describe our findings in 200 consecutive breast specimens evaluated for the presence of PHMS. The first patient presented with peau-de-orange change in the overlying breast skin, thus mimicking inflammatory breast carcinoma. Furthermore, this patient's PHMS lesion had been diagnosed and treated inappropriately as a low-grade angiosarcoma. The second case showed the more typical, fibroadenoma-like presentation of PHMS. In addition, PHMS changes occur commonly in routine breast biopsy specimens. In fact, our review of 200 consecutive breast specimens showed PHMS in at least one microscopic focus in 23% of cases. The PHMS changes occurred in younger patients than the control population and were associated with fibrocystic changes, in fibroadenomas, in gynecomastia, in normal breast tissue, and in sclerosing lobular hyperplasia. Ultrastructural and immunohistochemical studies of one case showed that the capillary-like spaces were either acellular or lined by fibroblasts. Pseudoangiomatous hyperplasia of mammary stroma represents a clinicopathologic spectrum, extending from focal, insignificant microscopic changes to cases where PHMS produces a breast mass. Increased awareness of PHMS and its clinicopathologic spectrum will allow its differentiation from other vascular tumors of the breast, especially low-grade angiosarcoma.