Hemangiopericytoma, an uncommon neoplasm derived from pericytes, occurs in many locations throughout the body. The clinical course of this tumor is variable with the most malignant lesions capable of producing metastases. This report describes 5 patients who had hemangiopericytoma of the breast. All were women between 47 and 57 yr old. The tumors varied from 3.2 to 19 cm (average, 9 cm). Three were treated by mastectomy and two by local excision. All remained disease free for a median duration of 22 mo and for an average of 46 mo (3 to 144 mo). We have found eight additional already published examples of mammary hemangiopericytoma in adult women ranging in age from 33 to 67 yr (average, 51 yr). The tumors averaged 6 cm (1 to 29 cm) in diameter. Three were treated by mastectomy and 5 by excision. All 8 patients remained disease free at last follow-up, averaging 84 mo (16 to 276 mo). Thus, it appears that hemangiopericytoma arising in the breast is a clinically low-grade form of sarcoma regardless of tumor size. Whenever possible, initial treatment should be complete local excision with breast preservation. Total mastectomy may be necessary for exceptionally large lesions.