Grossly circumscribed, nonhemorrhagic breast masses consisting of mammary stromal proliferations that simulated vascular lesions were studied in nine women. Histologically, a striking pattern, which appeared to consist of complex inter-anastomosing channels lined by slender spindle cells, was present in the mammary parenchyma. The importance of this benign lesion, referred to as pseudoangiomatous hyperplasia of mammary stroma, is its distinction from angiosarcoma. The patients ranged in age from 22 to 52 years; all were premenopausal. Each presented with a palpable unilateral mass, measuring up to 7 cm in diameter. The patients were treated by excisional biopsy and remained well for up to 2.5 years after excision. One patient had two local recurrences within one year of the original excision, and a second patient had a local recurrence at 14 months. No patient had another concurrent or metachronous malignant tumor of the breast or other organ, and no abnormal hormonal status was found. Complete local excision appears to be adequate treatment. It remains to be determined whether this is a neoplastic process. However, there is no evidence that it is a precursor of angiosarcoma, and ultrastructural observations demonstrate that the spaces found in the lesion are not true vascular channels. Rather, they appear to arise by a process that involves disruption and separation of stromal collagen fibers. Since small foci of this change are common in hyperplastic breast tissue from premenopausal women, it is likely that the development of a discrete tumor with this pattern represents an exaggerated form of stromal hyperplasia.