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Persistent eosinophilia can cause cardiac tissue damage, typically in the form of eosinophilic myocarditis, whether the underlying cause is reactive, a clonal myeloid disorder, or idiopathic hypereosinophilic syndrome (HES). Eosinophilic myocarditis ranges from mild localized disease to multifocal widespread infiltrates associated with myocardial necrosis, thrombotic complications and endomyocardial fibrosis. Systemic treatment varies widely depending on the underlying cause, so thorough investigation and precise diagnosis are essential. Evaluation includes assessment for reactive causes of eosinophilia (vasculitis such as eosinophilic granulomatosis and polyangiitis or Churg-Strauss, parasitic infection, autoimmune disease, immunoglobulinG4-related disease, medications and other causes), genetic lesions characteristic of clonal myeloid disorders (platelet-derived growth factor receptor-α, platelet-derived growth factor receptor-β and fibroblast growth factor receptor 1) and flow cytometry and molecular studies for the aberrant T cells characteristic of lymphocyte-variant HES. Patients with reactive eosinophilia require treatment for the underlying cause, such as antiparasitic therapy for helminthic infection or immunosuppression for eosinophilic granulomatosis and polyangiitis or Churg-Strauss. Those with a myeloid clone often benefit from the tyrosine kinase inhibitor imatinib. Steroids are the first-line treatment for idiopathic HES and lymphocyte-variant HES, and hydroxyurea or (pegylated) interferon-α may be used for relapsed or refractory disease. Mepolizumab, an anti-interleukin-5 monoclonal antibody, is an effective steroid-sparing agent in HES but is not widely available for this indication.