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Background: Polycythemia vera (PV) and essential thrombocythemia (ET) are associated with a well-recognized increased risk of thrombotic events, bleeding, and all-cause mortality, but the frequency of these outcomes during treatment has rarely been assessed in large cohorts.

Methods: In this nationwide, population-based study, we analyzed 2604 PV and 3141 ET patients using multiple Swedish health care registers, covering 43 612 patient-years. Rates of arterial and venous events, major bleeding, and all-cause mortality (ACM) were evaluated across therapies.

Results: Unexpectedly, 42.3% of low-risk PV patients and 29.7% of very low/low-risk ET patients received hydroxyurea (HU) during follow-up. High-risk PV patients treated with interferon (IFN) exhibited the lowest arterial event rate (2.21 per 100 patient-years). In high-risk ET, patients with IFN therapy experienced the lowest arterial and venous event rates (1.55 and 0.44 per 100 patient-years). Advanced age and leukocytosis at diagnosis independently predicted thrombosis, bleeding, and ACM in both PV and ET. Multivariable analysis showed that HU and IFN were associated with reduced ACM risk; HU also conferred protection against thromboembolism and major bleeding.

Conclusion: This study highlights risk factors associated with complications during treatment in a real-world context and reinforces the role of HU and IFN as first-line therapies in PV and ET.