A rare case of a mixed endometrial stromal and smooth muscle tumor arising in the uterus of a 74-year-old woman is reported. The patient underwent hysterectomy for an enlarging uterine mass, and a large intramural tumor, showing marked central hyaline necrosis with calcification, was found. The tumor consisted of an admixture of a low-grade endometrial stromal sarcoma (ESS) and a fascicular proliferation of spindle cells suggesting smooth muscle differentiation, and a characteristic ‘star-burst’ appearance was found. In the ESS region, there were a few small foci of anaplasia where large polygonal cells with atypical nuclei and abundant eosinophilic cytoplasm proliferated, and the proliferative activity was locally increased in these foci. A small metastatic nodule appeared in the lung nine months after the hysterectomy, and the resected metastatic lesion showed features of anaplastic spindle cell sarcoma which was immunoreactive for CD10 but not for smooth muscle markers. Mixed endometrial stromal and smooth muscle tumors should be regarded as malignant neoplasms with the potential for hematogenous metastasis, particularly when they contain foci of cellular anaplasia.