INTRODUCTION: Thrombosis is a leading cause of mortality in polycythemia vera (PV), yet little is known about early predictors of thrombosis progression. Emerging evidence links systemic inflammation to thrombosis risk. This study assessed hematological inflammatory parameters, including the neutrophil-to-lymphocyte ratio (NLR), monocyte-to-lymphocyte ratio (MLR), platelet-to-lymphocyte ratio (PLR), systemic immune-inflammation index (SII), systemic inflammatory response index (SIRI), and aggregate index of systemic inflammation (AISI), for their utility in predicting thrombosis progression in PV. METHODS: This retrospective study analyzed 102 PV patients (2008-2024) and 115 healthy controls. Clinical and laboratory data, including thrombotic events, were examined. Patients were stratified into low-and high-risk groups, further categorized by thrombosis progression. Inflammatory biomarkers were derived from baseline blood counts. RESULTS: Median follow-up was 51 months, the 10-year thrombotic events-free survival rate was 84.30%, and 21 patients (20.59%) showed thrombosis progression. PV patients had higher NLR, MLR, PLR, SII, SIRI, and AISI than controls. NLR, MLR, SIRI, and AISI were associated with an increased risk of thrombosis compared to the low-risk group. In the progression group, high monocytes, NLR, MLR, SIRI, AISI, and previous thrombosis were notably correlated. ROC analysis showed that MLR (AUC = 0.739), SIRI (AUC = 0.714), AISI (AUC = 0.670), and NLR (AUC = 0.649) had the highest predictive ability for thrombosis progression. Multivariate analysis identified MLR (HR = 6.979) and previous thrombosis (HR = 4.494) as independent risk factors. CONCLUSION: These findings support recent reports linking systemic inflammation to thrombosis risk in PV patients and highlight for the first time that MLR may serve as a valuable prognostic biomarker for thrombotic events.