Giant cell myocarditis is a rare but devastating disease that usually affects young otherwise healthy individuals. Associations with thymoma, inflammatory bowel disease, and a variety of autoimmune disorders have been reported. The rate of death or heart transplantation is approximately 70% at 1 year. Data from a Lewis rat model and from observational human studies suggest that giant cell myocarditis is mediated by T lymphocytes and may respond to treatment aimed at attenuating T cell function. Recent findings from the Giant Cell Myocarditis Registry, a clinical and pathologic database from 63 cases of giant cell myocarditis gathered from 36 medical centers, include the following: The sensitivity of endomyocardial biopsy for giant cell myocarditis for patients who undergo transplantation or autopsy is 82 to 85%. Registry subjects who received cyclosporine in combination with steroid, azathioprine, or muromonab-CD3 have prolonged transplant-free survival (12.6 months vs. 3.0 months for no immunosuppression). Post-transplantation survival is approximately 71% at 5 years despite a 25% rate of giant cell infiltration in the donor heart. To confirm and extend these findings, a randomized trial of immunosuppression including muromonab-CD3, cyclosporine, and steroids is underway.