Although carditis associated with Mycoplasma pneumoniae is infrequent it is an important cause of death in M. pneumoniae infections. We report on a 4-year-old boy with a M. pneumoniae infection, who developed a large hemorrhagic pericardial effusion and was successfully treated by percutaneous catheter drainage. Except in cancer patients, nontraumatic hemorrhagic pericardial effusions are rare. Our case strongly supports direct bacterial invasion into pericardial tissue as a cause of M. pneumoniae pericarditis rather than autoimmune phenomenon.