A nine and a half year old boy was brought to the hospital because of a cough, dyspnea and mild fever. He was well-nourished and had an uneventful history. His chest X-ray and electrocardiographic findings suggested pericarditis but further examinations and an open pericardial biopsy revealed a mass histologically diagnosed as pericardial mesothelioma, a very rare tumor in this age group.