Giant Cell Myocarditis is a rare but devastating disease that usually affects young otherwise healthy individuals. The rate of death or heart transplantation is approximately 70% at one year. Data from a Lewis Rat model and from observational human studies suggest that GCM is mediated by T lymphocytes and may respond to treatment aimed at attenuating T cell function. The Giant Cell Myocarditis Registry is a clinical and pathologic database from 63 cases of giant cell myocarditis gathered from 36 medical centers. Findings from the registry include the following: The sensitivity of endomyocardial biopsy for GCM for patients who undergo transplantation or autopsy is approximately 80%. Registry subjects who received cyclosporine and/or azathioprine, with steroid and sometimes muromonab-CD3 had prolonged transplant-free survival (12.6 months vs 3.0 months for no immunosuppression, Cooper, et al, NEJM 1997). Post-transplantation survival is approximately 71% at five years despite a 25% rate of giant cell infiltration in the donor heart. To confirm these findings, a randomized trial of immunosuppression including muromonab-CD3, cyclosporine, and steroid is underway. Participation in the Multicenter Giant Cell Myocarditis Treatment Trial from medical centers with experience in the diagnosis and treatment of Giant Cell Myocarditis is encouraged.