A case of a false aneurysm arising at the proximal suture of an aortic root replacement with a pulmonary autograft is presented. This complication did not occur in the first postoperative month but was discovered late, and the female eight-year-old patient was in an extremely serious condition. She was reoperated on an emergency basis but died of acute pulmonary artery hypertension. The mechanism of the occurrence of such a case is discussed. In the absence of infection, structural weakness of the right ventricular muscle with progressive tearing is suggested. Strict and prolonged echocardiographic surveillance after the Ross procedure and early reoperation are mandatory.