Introduction: Rheumatoid arthritis (RA) is an inflammatory, autoimmune disease with preferential involvement of the synovium in the proximal joints of the hands and feet. Formerly known as chronic progressive polyarthritis or chronic rheumatoid arthritis, its course is characterised by joint erosion and destruction. Coronary heart disease and strokes, the consequences of atheromatous disease, are part of rheumatoid arthritis and are responsible for the increased morbidity and mortality observed in this disease. Cardiac tumours in children are rare. The frequent use of echocardiography has helped to detect them earlier and with greater sensitivity. Most cardiac tumours in children are congenital, primary and benign. Most are discovered in children under one year of age. The aim of this study is to report the discovery of this type of tumour in a 10-year-old child being treated for seronegative polyarthritis. Observation: We report the observation of a 10-year-old girl, from a non-consanguineous marriage, with a history of systemic arthritis on difal 25mg *2/d + MTX 0.8mg/kg/week. Present since the age of 3 with recurrent arthralgias, admitted for polyarthralgias, fever with chest pain and whose examination found swelling, pain and limitation of passive and active movement of the left knee joint and elbow joint. Biological work-up revealed inflammatory microcytic hypochromic anaemia, normal ferritin, elevated CRP and ESR, ECBU, stool culture, ASLO serology, sputum BK, sputum TB PCR, Gene Xpert in sputum, Quantiiferon, SAM work-up and rheumatoid factors (RF) were negative, antinuclear antibodies (ANA) positive. EX. Ophthalmology and abdominal ultrasound normal, chest X-ray reveals cardiomegaly (ICT 0.6), X-ray of the spine + knee + elbow showed a discreet increase in the transparency of the bone structure. ETT found an oval mass measuring 20/25mm at the level of the junction between OD and VD not managing the reflux of the tricuspid valve and THORACIC CT in favour of a mass adherent to the pericardium evoking in the first instance a teratoma. Conclusion: Cardiac tumours in children are rare, and pericardial teratomas are often benign. In this study, we emphasise the importance of looking for underlying cardiac disease in the presence of recurrent episodes of arthralgia in children. Early positive diagnosis of the disease and the therapeutic implications are vital.