We report on a 45-year-old man with a confirmed diagnosis of acute myopericarditis associated with Mycoplasma pneumoniae. He visited our emergency department due to high fever (39degrees C) via a primary clinic. We made a diagnosis of myopericarditis based on symptoms, cardiac enzymes, electrocardiography, and transthoracic echocardiography. Serology (particle agglutination) testing for M. pneumoniae IgG antibody was also performed. The IgG antibody titer was 1:80 on the second day of admission, and increased to 1:2,560 by the 12th day of admission. Therefore, we confirmed the diagnosis of acute myopericarditis associated with M. pneumoniae and subsequently treated him with azithromycin. The symptoms and laboratory findings improved, and he recovered uneventfully.