Eosinophilic phenotypes in polycythemia vera (PV) and essential thrombocythemia (ET) are rare and poorly characterized. Co-occuring JAK2 mutations in cis, specifically both L611S or N622Y mutations, appear to result in a more aggressive clinical phenotype. PV/ET with eosinophilic phenotypes may require full next generation sequencing to capture co-occurring mutations as opposed to more prevalent single gene assays. These eosinophilic phenotypes are highly thrombotic and systemic symptoms appear responsive to early use of the janus kinase (JAK) inhibitor Ruxolitinib ¹⁾.