The renin-angiotensin system contributes to cell growth, proliferation, and differentiation in the bone marrow. We investigated the role of the ACE I/D gene polymorphism in 108 polycythemia vera (PV) and essential thrombocytosis (ET) patients who were positive for the JAK2V617F mutation, with a thrombosis group (TG) of 95 patients who had a history of vascular events, but did not have a history of myeloproliferative neoplasms and compared these to a healthy control group (CG) of 72 subjects. In the patients, II genotype and I allele frequency (p=0.009, odds ratio [OR]=9.716, 95% confidence interval [CI]=1.242-76.00, p=0.004, OR=2.019, 95% CI=1.243-3.280, respectively) were found to be higher than those in the controls. The DD genotype (p=0.021, OR=0.491, 95% CI=0.268-0.899) and D allele (p=0.004, OR=0.495, 95% CI=0.305-0.805) were found to be correlated with a decreased risk of a myeloproliferative neoplasm. These findings support the hypothesis that the ACE II genotype and I allele may be related to increased risk of ET and PV. Conversely, the DD genotype and D allele may be related to decreased risk of ET and PV. The results also indicated that the ACE I/D gene polymorphism was independent of thrombosis formation.