A fine structure of the myocardium was examined in the acute febrile mucocutaneous lymph node syndrome, which was recently prevailing in Japan. The right ventricular myocardial cells in ten cases were observed through endomyocardial biopsies. On light microscopy, accumulation of histiocytes was demonstrated in some cases. Electron microscopic examinations revealed myofibrillar changes, increased glycogen contents and other degenerative changes. On the myocardium of a case in which a biopsy was performed during the active phase of the disease, these changes were prominent. However, these changes were also observed for some months to some degree.