Background <p>Eosinophilic endocarditis or myocarditis is a major complication of hypereosinophilic syndrome, characterized by eosinophilic infiltration leading to endocardial or myocardial necrosis, thrombosis formation, and fibrosis. For its rare morbility and various heterogenicity, eosinophilic endocarditis or myocarditis is prone to misdiagnosis and missed diagnosis. Neither large case series nor clinical trials on this specific endocarditis or myocarditis have been reported.</p> Case presentation <p>Four middle-aged male patients had increased eosinophilia and elevated levels of troponin or lactate dehydrogenase. Cardiac ultrasound showed ventricular wall thickening with or without reduced cardiac systolic function, apical thrombosis or restrictive cardiomyopathy.one of these patients showed myocardial enhancement by CMR, and one of these patients showed endocardial enhancement by CMR. The coronary angiography results were negative. Three patients were diagnosed with eosinophilic endocarditis, and one was diagnosed with eosinophilic myocarditis. After the application of steroid treatment, eosinophil levels decreased rapidly, myocardial thickening was relieved, and cardiac function was gradually recovered.</p> Conclusion <p>This case series embodies the high heterogeneity in the clinical manifestation of the eosinophilic myocarditis or endocarditis, and the important role of multi-module imaging. Early detection and early treatment is crucial for the prognosis of eosinophilic endocarditis or myocarditis.</p>