Despite well-established criteria, the distinction of uterine leiomyosarcoma from certain variants of benign leiomyoma, particularly bizarre leiomyoma, can be challenging morphologically. Recent studies reported the overexpression of p16 protein in uterine leiomyosarcoma. However, the potential role of immunohistochemistry of p16, p53, and Ki-67 in the differential diagnosis of leiomyosarcoma and bizarre leiomyoma has not been well assessed. We had immunohistochemically studied the expression of p16, p53, and Ki-67 proteins in 100 cases of uterine smooth muscle tumors, including 35 usual leiomyomas (LM), 13 cellular leiomyomas (CLM), 15 bizarre leiomyomas (BLM), 2 cases of smooth muscle tumor of uncertain malignant potential (STUMP), and 35 leiomyosarcoma (LMS). For p16 immunostain, strong and mediate-to-diffuse staining pattern (>25% cells positive) was seen in 100% of LMS and STUMP. There were 86.5% of BLM that showed positive for p16, which includes 60% of them showing mediate-to-diffuse p16 positivity. Although 38.5% of CLM and 14% of usual LM showed p16 positivity, the majority of those (70%) are focal with weak p16 staining pattern. The strong, mediate-to-diffuse p53 immunostain was seen in 91% of LMS, 60% of BLM, and 50% of STUMP, but none of usual LM and CLM. More than 10% of cells positive for Ki-67 were observed in 83% of LMS, 100% of STUMP, and 48% of BLM, but none of usual LM and CLM. Our study indicates that distinct expression patterns for p16, p53, and Ki-67 exist between leiomyosarcoma and usual LM and CLM (P<0.0001). Immunohistochemical study with a panel of antibodies to p16, p53, and Ki-67 is helpful in distinguishing LMS from CLM and usual LM. However, due to significant overlapping staining patterns between LSM and BLM (P=0.09), immunostains for p16, p53, and Ki-67 have limited role in differentiating LMS from BLM.