Introduction: Uterine leiomyosarcoma (ULS) is a highly aggressive and lethal tumor. The absence of specific symptoms and diagnostic imaging findings makes the diagnosis of ULS challenging. Endometrial sampling reportedly has a significantly lower predictive value in diagnosing ULS compared with epithelial uterine malignancies. Materials and Methods: The authors retrospectively reviewed the preoperative endometrial cytology findings in seven of the 12 patients with ULS who were treated in this institution between 2008 and 2017. The other five patients did not have preoperative samples obtained for cytology. Results: Only one of the seven patients was originally diagnosed with a malignant tumor, with preoperative cytology showing rounded tumor cells with enlarged, irregular, hyperchromatic, and sometimes multiple nuclei, with conspicuous nucleoli in a necrotic background. After re-evaluation of the preoperative specimens, atypical cells were detected in four of the six patients who were initially deemed to have negative findings. A monomorphic population of spindle cells with slightly hyperchromatic, elongated nuclei was detected in two patients. Another two patients had rounded tumor cells with a moderate amount of basophilic cytoplasm and round or oval nuclei with minimal atypia. Conclusion: Novel diagnostic techniques are needed to accurately identify ULS in the preoperative period. Careful microscopic observation of the entire cytological specimen, together with detailed patient medical information, are essential to making a correct preoperative diagnosis of ULS.