Thrombocytosis (platelet count greater than 400,000/mm3) has been identified as a poor prognostic factor in many cancers. The objectives of the present study were to determine the incidence of thrombocytosis in advanced epithelial ovarian cancer and the cut off point level of platelet count in Thai patients determining thrombocytosis condition and association between survival time in both thrombocytosis condition (platelet count greater than 400,000/mm3 and 305,000/mm3). Descriptive study. One hundred and twenty epithelial ovarian cancer patients were included between January 2004 and December 2005. Their medical records were retrospectively reviewed and data was analyzed using chi2, Fisher's exact test, and Student t- test. The level of platelet counts determining thrombocytosis condition in Thai patients were record and analyzed by ROC curve. Survival was analyzed by the method of Kaplan and Meier. When using the standard criteria of thrombocytosis (platelet counts greater than 400,000/mm3), the incidence was 35% (42/120 cases). Among advanced-stage patients, such incidence was 56.7% (42/74 cases). The criteria for diagnosis of thrombocytosis in the present study was platelet counts greater than 305,000/ mm3. When using such criteria, the overall incidence of thrombocytosis was 49.1% (59/120 cases) and was 72.97% (54/74 cases) in advanced stage. Patients with preoperative thrombocytosis were found to have higher CA - 125 level, greater prevalence of ascites, and worse prognosis. The incidence of thrombocytosis in Thai patients with advanced epithelial ovarian cancer was 56.7% and 72.9% when using platelet count level greater than 400,000/mm3 and 305,000/mm3 respectively, which was higher than that of the early stage. This implies that thrombocytosis is probably a marker of cancer aggressiveness.