Primary sarcoma of the ovary is rare, and only 16 reported examples of leiomyosarcoma are known to us. We encountered a primary ovarian leiomyosarcoma in a 58-year-old woman. The tumor was characterized by interlacing bundles of plump spindle cells that showed immunoreactivity for desmin, vimentin, and muscle-specific actin. Ultrastructural features included abundant smooth-muscle-type filaments, notched nuclei, pinocytotic vesicles, and extracellular collagen. This case illustrates the utility of ancillary techniques in the classification of pure ovarian sarcomas and leads us to propose ultrastructural and immunohistochemical criteria for primary ovarian leiomyosarcoma.