Nearly 100 vascular tumors of the breast have been studied. Sixty-two were angiosarcomas lesions. Twenty-four patients with mammary hemangiomas are the subject of this report. Five of 11 microscopic perilobular hemangiomas, not clinically apparent (2 mm or less), were atypical and had nuclear hyperchromasia or focal anastomoses among vascular channels. Whether treated by excision or mastectomy, all 11 patients remain well with follow-up of up to 10 years. Fourteen clinically or grossly apparent macroscopic hemangiomas (0.3-2.5 cm) included eight characterized as cytologically atypical. With one exception, lesions designated hemangiomas were well circumscribed and tended to be divided into lobules. In more than half, origin from large, non-neoplastic "feeding" vessels that were seen branching into the lesion was demonstrated. Whether treated by excision or mastectomy, no hemangioma has recurred after follow-up, up to 5 years in some cases. The diagnosis of vascular tumors of the breast requires thorough microscopic study of the entire lesion. The majority are angiosarcomas. However, about one-third constitute a spectrum of apparently benign and atypical lesions, in which the single largest group are perilobular hemangiomas and hemangiomas. Size appears to be an important characteristic for distinguishing hemangiomas from angiosarcomas, as few lesions larger than 2 cm qualify as hemangiomas, whereas angiosarcomas are rarely smaller than 2 cm. It is possible that atypical perilobular hemangiomas or atypical hemangiomas are precursors to angiosarcoma, but this relationship remains to be demonstrated.