In this study, the role of the count of intratumoral mast cells was examined and compared with the proliferative activity exhibited by Ki-67 indices in the differential diagnosis of uterine smooth muscle tumors. Sixteen cases of leiomyosarcoma, nine cases of atypical leiomyoma and 16 cases of ordinary leiomyoma were included. The pathological features of the cases were determined by reviewing the archive materials including the patient records and hematoxylin-eosin-stained sections. Toluidine blue stain was used to highlight the intratumoral mast cells and they were counted in at least 40 high power fields. A standard streptavidin-biotin method was applied to the sections to highlight the Ki-67 immunoreactive tumor cell nuclei. These proliferative cells were counted in at least 10 high-power fields. Atypical leiomyomas tended to have a higher quantity of intratumoral mast cells than leiomyosarcomas and ordinary leiomyomas (P = 0.027 and P = 0.021, respectively). Leiomyosarcomas tended to have higher Ki-67 immunoreactivity rates than atypical leiomyomas, although the difference was not statistically significant (P = 0.82). We concluded that the quantity of intratumoral mast cells is useful in the differential diagnosis between leiomyosarcomas and atypical leiomyomas, while the cell proliferation rate expressed by Ki-67 immunoreactivity has a limited value.