Background: Thrombotic events in essential thrombocythaemia (ET) are difficult to predict with current risk stratification based on age and prior history of thrombosis. Aims: We aimed to assess the predictive value of the JAK2 V617F mutation (JAK2) and spontaneous erythroid colony (SEC) growth for the development of thrombotic events post diagnosis in patients with ET. Methods: Consecutive patients with ET were retrospectively identified, and clinical and laboratory correlates were evaluated. Thrombotic events were categorized according to their occurrence at or prior to diagnosis (prior thrombosis), and any time post diagnosis of ET (subsequent thrombosis). JAK2 analysis was performed by allele-specific PCR on whole blood or bone marrow. Results: A total of 62 patients was identified, median age 63years; 67% (41/61) JAK2-positive and 47% (25/53) SEC-positive. Median follow-up was 33months (range, 1 to 137). JAK2-positive patients showed a trend to increased prior thrombosis (27% vs 5%, P= 0.08), and a significant increase in the development of subsequent thrombosis (5-year event rate 31% vs 6%, P= 0.04), which persisted when stratified for a history of prior thrombosis (P= 0.04). Survival was not affected by JAK2 status. The SEC assay predicted an increased rate of baseline thrombosis (16% vs 0%, P= 0.04), but was not found to be predictive of any subsequent thrombotic events. Conclusions: Patients with ET who are JAK2-positive by whole blood allele-specific PCR appear to be at increased risk of thrombotic complications, which is independent of a prior history of thrombosis.