AbstractHypereosinophilic Syndrome (HES) is caused by the uncontrolled proliferation of eosinophils generally associated with conditions such as allergic reactions or parasitic infections. This syndrome is characterized by excessive eosinophil production (>1500/mm3) that persists for more than six months and cannot be explained by secondary causes. HES symptoms can affect different body organs, and usually, nonspecific symptoms include fever, malaise, fatigue, rash, shortness of breath, and myalgia.HES is a rare disease with multiorgan involvement, including the skin, joints, kidneys, vascular system, gastrointestinal tract, cardiac and pulmonary systems. The main feature of this disease is that overproduced eosinophils accumulate in organs and cause organ damage. Cardiac involvement plays a critical role in determining morbidity and mortality, and cardiac and large vessel thrombosis with severe clinical manifestations can also be observed.Treatment aims to reduce the absolute eosinophil count, improve symptoms, and prevent disease progression. Pharmacologic therapy aims to maintain targeted eosinophil levels below 1.5 x 10^9/L (1500 cells/mcL) to reduce the symptoms of eosinophilic disease and prevent organ damage. Furthermore, indications for emergency treatment should be rapidly assessed and initiated promptly in appropriate patients.This paper will discuss the diagnosis, clinical manifestations, treatment modalities, and management challenges of HES in detail through two rare case examples.