Giant cell myocarditis is a rare and frequently fatal disorder of unknown origin that is defined histopathologically as diffuse myocardial necrosis with multinucleated giant cells in the absence of sarcoidlike granulomata. The clinical and pathologic features of lymphocytic myocarditis have been described in several recent publications, but the features of idiopathic giant cell myocarditis have not been adequately addressed.We describe five patients with idiopathic giant cell myocarditis who were seen at Stanford University over the past 10 years. In each case the onset was subacute congestive heart failure. After diagnosis each patient received immunosuppressive therapy and was evaluated for heart transplantation. Progressive heart failure and ventricular arrhythmias developed in all. Three died rapidly, two of progressive heart failure and one of sudden cause. Two patients underwent orthotopic heart transplantation and are currently alive, one with disease recurrence. Pathologic studies, including endomyocardial biopsy and evaluation of postmortem or explanted material at transplantation were reviewed. The pathologic studies provided additional support that the giant cells derive from a monocytic/histiocytic lineage. Segmental wall motion abnormalities suggest giant cell myocarditis can be a focal, as well as diffuse process at certain stages of its course. This experience is compared with published cases and implications for diagnosis and treatment are discussed.In view of the uniformly fatal nature of the disease, heart transplantation should be a serious consideration, and the patients evaluated once the diagnosis is established. Triple-drug immunosuppressive therapy should be considered at the time of diagnosis.