In patients with Philadelphia-negative chronic myeloproliferative neoplasms (MPNs), the anti-thrombotic and/or cytoreductive treatment in the follow-up may affect the evaluation of the pro-thrombotic weight of the clinical and biological characteristics at diagnosis. In order to avoid this potential confounding effect, we investigated the relationship between prior thrombosis (PrTh: thrombosis occurred before diagnosis and before treatment) and the characteristics at diagnosis in 977 thrombocythemic patients with MPN, reclassified according to the WHO 2008 criteria. PrTh occurred in 194 (19.9%) patients, with similar rates in the different MPNs. In multivariate analysis, PrTh rate was significantly related to minor thrombocytosis (platelets10 x 10(9)/L), higher hematocrit (HCT >45%), JAK2 V617F mutation, older age, and cardiovascular risk factors (CVRFs). The highest PrTh rate (33.9%) was associated with the coexistence of minor thrombocytosis and leukocytosis. Of note, the inverse relationship between PrTh rate and platelet count is consistent with the hemostatic paradox of thrombocytosis. In conclusion, this analysis in MPN patients disclosed the unbiased characteristics at diagnosis with a pro-thrombotic effect. Moreover, it suggests that the optimal control of blood cells counts, and CVRFs might be of utmost importance in the prevention of thrombosis during the follow-up. (C) 2016 Elsevier Ltd. All rights reserved.