Mineralization in leiomyosarcoma, a malignant tumor of smooth muscle, has not been widely recognized. In this article, we report our experience with four cases of primary leiomyosarcoma of soft tissue or bone in which mineralization was visible on either radiography or CT. In none of the cases was the diagnosis of leiomyosarcoma considered before biopsy. In one case of a soft-tissue leiomyosarcoma, the presence of mineralization was a factor that led to the misinterpretation of the needle biopsy specimen as soft-tissue osteosarcoma. Histologically, mineralization in leiomyosarcoma appears to be caused by either nonneoplastic ossification or dystrophic mineralization in the tumor. This feature can cause leiomyosarcomas to be confused with other neoplasms.