We report a case of idiopathic giant cell myocarditis accompanied by asymmetric septal hypertrophy. A 64-year-old woman was admitted because of dyspnea. There was no past history of hypertension or heart disease and no family history of hypertrophic cardiomyopathy. Laboratory examinations revealed general inflammatory changes and mild elevation of serum CK and GOT. The clinical course was fulminant and the patient died of heart failure one day after admission. On autopsy, asymmetric septal hypertrophy was revealed and the pathohistological examination revealed panmyocarditis with mononuclear cell infiltration, interstitial edema, necrosis of myocytes, and giant cells. The inflammatory changes were most severe in the ventricular septum with asymmetric septal hypertrophy. The extent of myocardial fibers with disarray was within normal limits. Thus, the asymmetric septal hypertrophy appeared to be due to marked interstitial edema and inflammatory cell infiltration in the septum. This case suggests that myocardial inflammation and edema may cause thickening of the ventricular wall during the course of acute myocarditis.