We describe a patient with symptoms of heart failure caused by severe mitral regurgitation. Echocardiography revealed an intracardiac mass embedding the posterior mitral valve leaflet, and cardiac magnetic resonance imaging showed two intracardiac thrombi and endomyocardial fibrosis. Eosinophil count kept rising and a mutation in the gene for platelet-derived growth factor receptor alpha was found. The combination of these findings led to the diagnosis of Loeffler's endocarditis. Treatment with prednisone and a tyrosine kinase inhibitor resulted in complete remission of the hypereosinophilia and mitral valve regurgitation was only mild at 9-month follow-up visit. .