Purpose: We discuss the coexistence of Klippel-Trénaunay-Weber syndrome with various malignancies, the possible histogenetic pathways and therapeutic implications.Patient: We report on a 46-year-old man presenting with increasing pain and swelling of his right lower leg after fracturing his fibula. Since birth he was known as having the uncommon syndrome of Klippel-Trénaunay-Weber of his right lower leg.Methods: Our patient underwent an above-knee amputation for biopsy-proven malignant vascular tumour, first thought to be a composite hemangio-endothelioma and/or angiosarcoma with lung metastases.Results: In the amputated extremity, a vascular malformation was found with tumour showing various components with foci of angiosarcoma adjacent to diffuse neurofibroma and areas with high-grade malignant peripheral nerve sheath tumour. Amputation and palliative chemotherapy were indicated, but he died of pulmonary and cerebral metastases 2 months postoperatively.Discussion: This case describes an angiosarcoma with malignant peripheral nerve sheath tumour developing in a patient with Klippel-Trénaunay-Weber syndrome. A case never described before in literature and probably, as in our case, difficult to diagnose at first biopsy.