Angiosarcoma is an uncommon but aggressive tumor of endothelial origin that may occur in the upper extremities of patients with postmastectomy lymphedema (Stewart-Treves syndrome) as well as in other regions. We present an unusual case of angiosarcoma associated with congenital nonhereditary lymphedema in an 18-year-old man. Our case underscores the need for a careful clinical examination and shows the importance of appropriate sampling and thorough pathologic examination of suspicious areas to exclude the presence of a malignant process.