We report a rare case of giant uterine leiomyosarcoma in a postmenopausal woman, whose diagnosis was initially suspected at the evaluation of the abdominal efusion, and confirmed after the pathological examination of the uterus in association with the ancillary tests. The evaluation of the abdominal fluid showed single or clusters of malignant, round or spindle-shaped cells. On microscopic examination of the surgical specimen, a dense cell proliferation of spindle cells, with moderate to severe nuclear pleomorphism and significant mitotic activity was observed. Immunohistochemical evaluation demonstrated the loss of myocytic differentiation by focal, weakly positive expression of smooth muscle actin and desmin. The data presented in this case emphasize the relevance of the cytological examination, although the latter has only indicative value, especially since it is an aggressive tumor, frequently associated with mutant expression of p53. In our case, the first indication of the presence of uterine sarcoma was given by the presence of atypical cells in the peritoneal fluid.