This report describes three female patients who were treated for diffuse vascular lesions of the breast. The lesion was congenital in one patient. Two were adults (40 and 59 years old). Each presented with a mass within the breast. Microscopically, the tumors were composed of vascular channels growing diffusely in breast parenchyma. The vessels were lined by flat, inconspicuous endothelium with sparse supporting mural tissue virtually devoid of smooth muscle. The lesions consisted of hemangiomatous (erythrocyte-containing) and lymphangiomatous (seemingly empty) channels with lymphoid aggregates. This combination of a mixed pattern of vessels and diffuse growth is similar to that seen in lesions that have been called angiomatosis in other anatomic sites. Because the microscopic distinction between very orderly angiosarcomas and angiomatosis may be difficult, complete excision of the tumor is necessary. For bulky lesions, this may require mastectomy. In one patient congenital angiomatosis has persisted and recurred twice in the mammary region over 39 years. The other two patients remain well 3 years after local excision and 2 years and 4 months after mastectomy, respectively. Angiomatosis of the breast has not exhibited clinically malignant behavior since no metastases have been observed.