Stewart-Treves syndrome is a rare cutaneous angiosarcoma with a poor prognosis. Physicians must be aware of this lethal syndrome, especially in patients who have been treated for breast cancer with radiation and lymph-node dissection (such as the case reported here). Patients who develop unexplained enlarging plaques of coalescing purple papules should have immediate biopsy for early diagnosis of Stewart-Treves syndrome. MRI, along with PET/CT, can help evaluate the extent of disease and help with treatment strategies.