Giant cell myocarditis (GCM) is a rare and highly lethal disorder. The only multicenter case series with treatment data lacked cardiac function assessments and had a retrospective design. We conducted a prospective, multicenter study of immunosuppression including cyclosporine and steroids for acute, microscopically-confirmed GCM. From June 1999 to June 2005 in a standard protocol, 11 subjects received high dose steroids and cyclosporine, and 9 subjects received muromonab-CD3. In these, 7 of 11 were women, the mean age was 60 ± 15 years, and the mean time from symptom onset to presentation was 27 ± 33 days. During 1 year of treatment, 1 subject died of respiratory complications on day 178, and 2 subjects received heart transplantations on days 2 and 27, respectively. Serial endomyocardial biopsies revealed that after 4 weeks of treatment the degree of necrosis, cellular inflammation, and giant cells decreased (p = 0.001). One patient who completed the trial subsequently died of a fatal GCM recurrence after withdrawal of immunosuppression. Her case demonstrates for the first time that there is a risk of recurrent, sometimes fatal, GCM after cessation of immunosuppression. In conclusion, this prospective study of immunosuppression for GCM confirms retrospective case reports that such therapy improves long-term survival. Additionally, withdrawal of immunosuppression can be associated with fatal GCM recurrence.