Loeffler's endocarditis remains is a very rare disease, develops due to eosinophilic inflammation predominantly of the endocardium with an outcome in fibrosis and massive thrombus formation and. He is generally characterized by an unfavorable prognosis. Clinical case of a 42-year-old patient with Loeffler endocarditis is presented. The development of the disease was preceded by a polyvalent allergy, mild dry eye syndrome and pansinusitis with a single eosinophilia of blood up to 16%. The reason for the hospitalization was the appearance of biventricular heart failure. During the previous year, the level of blood eosinophils remained normal, a threefold increase in the level of eosinophilic cationic protein was observed once. A 20-fold increase in the pANCA level, a 2.5-fold increase in the level of antibodies to DNA, an antibody to the nuclei of cardiomyocytes 1:160 were detected. The diagnosis was made on the basis of electrocardiography data (low QRS voltage, atrial hypertrophy), echocardiography, multispiral computed tomography and magnetic resonance imaging of the heart (thickening and delayed contrasting of the endocardium, massive thrombosis of the left ventricular apex with obliteration of its cavity, encapsulated fluid in the pericardium with compression of the right ventricle). Systolic dysfunction, severe signs of restriction and arrhythmias were absent. Trombectomy, tricuspid valve plasty, pericardial resection, suturing of an open oval window were performed. Signs of active inflammation with single eosinophils, vasculitis, perimuscular sclerosis, endocardial sclerosis were detected in morphological and immunohistochemical studies of endo-, myo-, pericardium. Viral genome was not found. The therapy with methylprednisolone 24 mg/day, azathioprine 75 mg/day was started. Six months after the operation, the symptoms of heart failure are completely absent, the thrombosis did not recur.