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ABSTRACTCarcinosarcomas (CS) are biphasic tumors with malignant epithelial and mesenchymal elements. The sarcomatoid elements of CS can include chondrosarcoma, malignant fibrous histiocytoma, osteosarcoma, leiomyosarcoma, fibrosarcoma, or liposarcoma. CS of the upper urinary tract are extremely rare but are associated with a poor prognosis. We report a case of a 44-year-old man with a localized right renal pelvis mass treated with a right nephroureterectomy. The pathological examination showed a high-grade urothelial carcinoma of the renal pelvis, stage III (pT3aNxM0). A few days later, he developed lower back pain, hematuria, cough with hemoptoic sputum and progressive dyspnea. Radiological explorations showed multiple bilateral lung nodules and a retroperitoneal mass. A CT-guided biopsy of the retroperitoneal mass revealed a high-grade angiosarcoma. A review of the nephrectomy specimen showed a microscopic focus of angiosarcoma in the urothelial carcinoma. Therefore, the initial diagnosis was changed to CS of the renal pelvis with an angiosarcoma component. The patient developed progressive respiratory failure and died 8 weeks after surgery. An autopsy revealed a large retroperitoneal mass with metastatic nodules to the abdominal wall, diaphragm, small intestine, liver, spleen, and lung. All lesions were angiosarcoma, with no evidence of urothelial carcinoma. This is the first case reported of a patient with CS of the upper urinary tract with an angiosarcoma component with a very aggressive course that caused the immediate appearance of multiple angiosarcoma metastases. We also describe the clinical and molecular characteristics of CS, which will help to contribute to a better understanding of this type of tumor.