Objective. Thrombocytosis has been associated with higher stage and mortality of cancer, however, the evidence is conflicting.We examined the stage distribution and prognosis of gynecologic cancer according to levels of prediagnostic platelet count. Methods. In a primary care resourcewith blood cell counts frommore than 500,000 individuals, we identified 581 women with a primary diagnosis of gynecological cancer. We divided the pre-diagnostic mean platelet count derived from the 3-year period prior to cancer diagnosis into three categories of thrombocytosis (no, 150400 109/L; mild, N400550 109/L; severe, N550 109/L). Logistic regression models were used to calculate odds ratios (ORs) for the association of prediagnostic platelet counts with stage at diagnosis. Subsequently, we estimated hazard ratios (HRs) for all-cause or gynecological cancer-specific mortality by level of thrombocytosis using Cox proportional hazard regression models. Results. Patients with non-localized disease had higher levels of prediagnostic platelet count [mild thrombocytosis: OR, 2.36 (95% CI, 1.334.19); severe thrombocytosis: 4.54 (95% CI, 1.5513.3); compared with no prediagnostic thrombocytosis]. The median overall survivalwas 1.04 years among patientswith severe prediagnostic thrombocytosis and 3.25 years among those with mild thrombocytosis, P b 0.0001. When adjusting for stage at diagnosis, elevated all-cause mortality remained with both mild (HR=1.51; 95% CI, 1.082.12) and severe (HR=3.15; 95% CI 1.95-5.08) prediagnostic thrombocytosis. Similar associations were found for cancer-specific mortality. Conclusions. Prediagnostic thrombocytosis was associated with advanced stage of gynecological cancer at diagnosis and increased all-cause and cancer-specificmortality. The platelet countmay have an important role in diagnosis and post-diagnostic control of gynecological cancer.