Established risk factors for thrombosis in essential thrombocythemia (ET) include age (≥ 60 years) and previous vascular events. Recently, also leukocytosis has been proposed in risk stratification of ET patients. We report a retrospective study on 532 ET patients followed for a median of 7.6 years. Sixty-four patients (12%) developed 95 thrombotic events during follow-up. Together with the high-risk condition, a white blood cell (WBC) value above 11 × 10⁹/L, corresponding to the fourth percentile value, significantly correlated with a higher thrombotic risk (p = 0.033) by Cox proportional hazards. Moreover, the cumulative risk of thrombosis was significantly higher in high-risk patients with WBC >11 × 10⁹/L. JAK2 V617F mutation did not correlate with thrombosis. Overall, 123 (23%) patients died. Three independent parameters were noted as prognostic factors for survival in multivariate analysis: age > 60 years, leukocytosis >11 × 10⁹/L, and hemoglobin level below normal values. Based on these parameters, three groups of risk were defined, with significantly different survivals. Baseline leukocytosis correlated with a higher thrombotic risk in high-risk patients and identified a cohort of patients with worse survival.