We report a case of pericarditis occurring in a 17-year-old girl who received a bone marrow transplant. The presence of Mycoplasma pneumoniae in the pericardial fluid was demonstrated by polymerase chain reaction, and Mycoplasma-like organisms were detected by electron microscopy. Because no other infectious agent could be found in the pericardial fluid, M. pneumoniae was presumed to be the etiologic agent of the pericarditis.