A case of a vascular tumor clinically and pathologically consistent with acquired progressive lymphangioma (benign lymphangioendothelioma) in a 48-year-old woman is reported. The lesion appeared in the skin close to a mastectomy scar 3 years after surgery and radiotherapy for invasive ductal carcinoma. On histologic examination, it mimicked an aggressive vascular neoplasm because of its infiltrative pattern. However, follow-up studies confirmed the benign nature of the lesion, clinically and histologically. This case indicates that acquired progressive lymphangioma may follow radiotherapy and must be considered in the differential diagnosis of other vascular proliferations occurring in the skin of the breast, especially of low-grade postradiation angiosarcoma, a recently described neoplastic entity.