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Heterologous differentiation in metastatic leiomyosarcomas of uterine origin is an extremely rare phenomenon. We report a remarkable case of a metastatic leiomyosarcoma from the uterus with an unusual pattern of tumor progression to malignant mesenchymoma after chemotherapy. The patient, an 80-yr-old woman with a history of metastatic leiomyosarcoma of the uterus to the lungs, presented with a large intra-abdominal mass. Histologic examination of the intraperitoneal mass demonstrated a high-grade sarcoma containing various heterologous malignant mesenchymal elements including osteosarcoma, chondrosarcoma, a liposarcoma-like area, and osteoclast-like multinucleated giant cells. Only the identification of small areas of smooth muscle differentiation revealed the true nature of the tumor as a metastatic leiomyosarcoma with aberrant sarcomatous differentiation. This unique presentation emphasizes the importance of clinicopathologic correlation in the diagnosis of tumors with unusual histology.