A 66-year-old patient with post-pericardiotomy syndrome after aortic valve replacement is reported in the article. Echocardiography in a month after surgery revealed moderate pericardial effusion (up to 50 ml) and fibrin elements. In order to avoid constrictive pericarditis, prednisone was prescribed in addition to previous therapy at a dose of 30 mg/day for a month. However, even short-term course of hormonal therapy resulted deterioration of chronic heart failure, left ventricular dilatation up to 180 ml and reduced ejection fraction up to 44%. This effect was due to mineralocorticoid action of prednisolone. More aggressive therapy aimed at suppression of renin-angiotensin-aldosterone system was required. Sacubitril/valsartan at a dose of 100 mg twice a day was administered. After a month of treatment, cardiac hemodynamics was normalized (end-diastolic volume — 160 ml, ejection fraction — 53%). Thus, sacubitril/valsartan may be recommended for aggravation of chronic heart failure as more effective drug compared to angiotensin-converting enzyme inhibitors.