Hypereosinophilic syndrome is a heterogeneous group of disorders, the majority of which are idiopathic. Cardiac manifestations, particularly eosinophilic myocarditis and endomyocardial fibrosis, are a typical course of morbidity and mortality in hypereosinophilic syndrome. We present a case of a patient with asthma and idiopathic eosinophilia who presented with dyspnea and edema. Cardiac ultrasonography showed pericardial effusion and reduced left ventricular motion, which persisted despite heart failure therapy, although pulmonary congestion improved. The peripheral blood eosinophil count was markedly elevated four days after admission, even though eosinophilia was not present at admission. Parasitic disease, autoimmune disease, and drug-induced cardiomyopathy were excluded as possibilities. A high dose of steroid therapy was started due to eosinophilic myocarditis. Cardiac function improved soon after therapy, along with a reduction in eosinophils. Upon retrospective examination, cardiomegaly and low voltage were observed, along with an elevation in the eosinophil count 15 months before admission. Monitoring chest radiography and electrocardiograms according to fluctuations in eosinophils may enable early detection and treatment of cardiac involvement in patients with hypereosinophilic syndrome, as demonstrated by this case.