Myeloproliferative neoplasms (MPN) are uncommon in children/young adults. Here we present data on unselected patients diagnosed before 25 years of age included from 38 centres in 15 countries. Sequential patients were included. We identified 444 patients, with median follow up 9.7 years (0-47.8). Forty-nine (11.1%) had a history of thrombosis at diagnosis, 49 new thrombotic events were recorded (1.16 % pt/year), peri-hepatic vein thromboses were most frequent (47.6% venous events) and logistic regression identified JAK2V617F mutation (p=0.016) and hyperviscosity symptoms (visual disturbances, dizziness, vertigo, headache) as risk factors (p=0.040). New hemorrhagic events occurred in 44 patients (9.9%, 1.04 % pt/y). Disease transformation occurred in 48 patients (10.9%, 1.13 % pt/year), usually to myelofibrosis (7.5%) with splenomegaly as a novel risk factor for transformation in ET (p= 0.000) in logistical regression. Eight deaths (1.8%) were recorded, three after allogeneic stem cell transplantation. Concerning conventional risk scores: IPSET-T and IPSET-NT differentiated ET patients in terms of thrombotic risk. Both scores identified high-risk patients with the same median thrombosis-free survival of 28.5y. No contemporary scores were able to predict survival for young ET or PV patients. Our data represents the largest real-world study of MPN patients age <25 years at diagnosis). Rates of thrombotic events and transformation were higher than expected compared with the previous literature. Our study provides new and reliable information as a basis for prospective studies, trials, and development of harmonized international guidelines for the specific management of young patients with MPN.

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TP and CG contributed equally to this work REFERENCES SUPPORTING INFORMATIONAdditional supporting information may be found online in the Supporting Information section at the end of the article.

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