A 15-year-old male presented to the Pediatric Cardiology Department in January 2019 with dyspnea at rest accompanied by chest pain. Medical interview revealed that the patient was suffering from mild abdominal pain, headache and asthenia in last few days. Medical and family history was unremarkable, he was diagnosed with allergic rhinitis and scoliosis. He has never been chronically treated. Physical examination revealed a young Caucasian who was in an obvious acute distress. Vital signs at presentation were blood pressure 110/82 mmHg, heart rate 100/min, respiratory rate 24/min, temperature 36.6  C, oxygen saturation 99% without oxygen support, height 179 cm, weight 52 kg. Pertinent objective findings were muffled heart sounds, distention of the jugular veins, ascites with hepatomegaly and right lower quadrant tenderness on palpation with no guarding or rebound tenderness. Laboratory results showed white blood cells (WBC) 4,870/uL, hemoglobin 14 g/dL, hematocrit 44.7%, platelets 152.000/uL, glucose 124 mg/dl, international normalized ratio 1.3; C-reactive protein <5 mg/l, Troponin T 8.8 ng/l. Electrocardiogram revealed sinus tachycardia, small amplitude of QRS complex, ST elevation and depression of PQ segment in many canals. Echocardiography was performed, 40 mm pericardial fluid was depicted. Patient was diagnosed with heart tamponade. An emergency pericardiocentesis was performed, what is more, drainage tubes were inserted into the pericardial cavity. The samples of blood and purulent fluid were collected for microbiological tests. No germs developed in the bacterial examination of the pericardial fluid. Broad-spectered antibiotics were applied with colchicine, non-steroidal anti-inflammatory drugs and corticosteroids. Contrast-enhanced computed tomogram revealed pericardiac and bilateral pleural effusions, moreover an appendix with appendicolith (Fig. 1). After evaluation by a general surgeon, the appendectomy was postponed until the etiology of pericarditis would be clarified. Although, the patient was consulted by many specialists, none of them was able to disclose the ground of the patient's disease. Many laboratory tests were conducted. Not only contagious diseases were excluded, but also lymphoma, cystic fibrosis, connective tissue disorders, immune deficiency, hypothyroidism, coeliac disease. The patient's plasma level of antibodies was negative for numerous pathogens (aerobic and anaerobic bacteria, Mycoplasma tuberculosis, Toxoplasma gondii, Ebstein-Barr virus, Cytomegalovirus, hepatic viruses, Chlamydia trachomatis, yersinia enterocolica, Candida etc). Moreover, heart magnetic resonance imaging was performed and, based on the results, the patient was diagnosed with restrictive pericarditis (Fig. 1). After one month of hospitalization the inflammatory parameters increased (WBC 22.210/ul, C-reactive protein 14.1 mg/l, procalcitonin 0.18 ng/mL) additionally the hemodynamic parameters worsened. The patient subsequently underwent partial pericardectomy through anterolateral left thoracotomy performed by the an experienced professor of pediatric cardiac surgery. The clinical postoperative evolution was favorable, with normal echocardiographic aspect. The drainage tubes were extirpated. From the patient's point of view that period of hospitalization was the most stressful. He demanded some psychologists' help in aim to return to normal activity. 5 days after the patient had been discharged, he was readmitted to the Emergency Department, because of acute abdominal pain. Physical examination revealed right lower quadrant tenderness on palpation without vomiting, nausea or fever. The abdominal ultrasonography revealed prominent appendix with an appendicolith up to 25 mm consistent with probable acute appendicitis. In addition, echocardiography was performed and reported recurrence of pericardial effusion. After evaluation by a surgeon, the patient subsequently underwent laparoscopic appendectomy. The clinical postoperative evolution was favorable moreover the effusive fluid absorbed completely. The patient was discharged 4 days after the operation with signs of normal wound healing and continues to do well 1 year postsurgery. He does his regular check-ups in a medical clinic.