Background: Platelets (PLTs) are highly reactive cellular effectors of hemostasis, immunity, and inflammation. Recent clinical observation and experimental evidence suggests that PLTs may promote cancer progression through diverse mechanisms, including protection of cancer cells from immune surveillance, negotiation of cancer-cell arrest in the microvasculature, and stimulation of angiogenesis. We investigated whether PLT counts (especially, thrombocytosis) were related to cancer stages and clinical outcomes in ovarian cancer. Methods: We analyzed clinical data on 538 patients with epithelial ovarian cancer diagnosed in CHA Bundang medical center from 2000 to 2013. They were tested associations between platelet counts and clinicopathological variables using Mann-Whitney test and Fisher`s exact test. Thrombocytosis was defined as a platelet count of more than 450,000/mm2. Results: The median PLT count in patients with thrombocytosis was 521,000/mm2. Patients with advanced disease correlated with higher PLT count than those with stage I + II + III (median PLT counts 267,000 vs. 303,000/mm2, p=0.004). Seven percent of the patients (36 of 538) had thrombocytosis at the time of initial diagnosis of epithelial ovarian cancer. Patients with thrombocytosis (19% in stage IV, 6% in stage III, 9% in stage II, 3% in stage I) were significantly more likely to have advanced-stage disease (odd ratio=5.069, 95% CI 2.482-10.351, p<0.001; using Fisher`s exact test) and higher preoperative levels of median cancer antigen (CA) 125 than those with normal platelet counts (43.9 vs 600 ng/ml, p<0.001). Conclusion: Advanced-stage ovary cancers are associated with increased PLT counts. The potential clinical application of thrombocytosis for prognosis of clinical outcomes in ovary cancer patients needs further studies.