This study aimed to evaluate the prevalence of preoperative thrombocytosis and its prognostic significancein Thai patients with endometrial cancer. We retrospectively reviewed the medical records of 238 cases whohad undergone surgical staging procedures between January 2005 and December 2008. Associations betweenclinicopathological variables and preoperative platelet counts were analyzed using Pearson’s chi square or twotailedFisher’s exact tests. Survival analysis was performed with Kaplan-Meier estimates. Univariate and Coxregressionmodels were used to evaluate the prognostic impact of various factors including platelet count in termsof disease-free survival and overall survival. The mean preoperative platelet count was 315,437/μL (SD 100,167/μL). Patients who had advanced stage, adnexal involvement, lymph node metastasis, and positive peritonealcytology had significantly higher mean preoperative platelet counts when compared with those who had not.We found thrombocytosis (platelet count greater than 400,000/μL) in 18.1% of our patients with endometrialcancer. These had significant higher rates of advanced stage, cervical involvement, adnexal involvement, positiveperitoneal cytology, and lymph node involvement than patients with a normal pretreatment platelet count. The5-year disease-free survival and overall survival were significantly lower in patients who had thrombocytosiscompared with those who had not (67.4% vs. 85.1%, p=0.001 and 86.0% vs. 94.9%, p=0.034, respectively).Thrombocytosis was shown to be a prognostic factor in the univariate but not the multivariate analysis. Inconclusion, presence of thrombocytosis is not uncommon in endometrial cancer and may reflect unfavorableprognostic factors but its prognostic impact on survival needs to be clarified in further studies.