Purpose: Serous primary peritoneal carcinoma (PPC) is histologically identical and clinically similar to epithelial ovarian carcinoma (OvC). In OvC thrombocytosis was found to be a marker of advanced disease and poor prognosis in most studies. Thrombocytosis in PPC has hitherto not been assessed. The purpose of the present investigation was to assess the prevalence of thrombocytosis in PPC patients, its association with prognostic factors, and survival. Material and Methods: The pretreatment thrombocyte count and selected clinical data in Stage IIIC histologically confirmed consecutive PPC patients, and a comparison group of Stage IIIC OvC patients diagnosed during January 2004 to December 2015, were abstracted from medical records. Results: The study included 21 PPC and 42 OvC patients. The rate of thrombocytosis in PPC patients was significantly higher (57.1% vs. 16.7%; p <0.001) and the five-year survival was lower than that of OvC patients. Among the PPC patients, there was a considerable, statistically non-significant, higher rate of grade 3 tumors and of patients with postoperative residual disease in those with thrombocytosis compared to those without it. A borderline significantly higher rate of < 65 years old patients was found in those with thrombocytosis compared to those without it. The five-year survival rate was not different among those with and those without thrombocytosis. Conclusion: The rate of thrombocytosis is higher in PPC than in OvC patients. In spite of the considerable excess of PPC patients with poor prognostic factors in those with thrombocytosis, their survival was not different than of those without thrombocytosis.