Active myocarditis, a common precursor of dilated cardiomyopathy, is defined as myocardial inflammation and injury in the absence of ischemia. Many agents may cause myocarditis and the exact manifestation of the disease depends on the interplay between the inciting agent and host response. Widespread belief that myocarditis is an autoimmune disorder has led to the possible overuse of endomyocardial biopsy and immunosuppressive agents. Their use is further complicated by the observation that many patients improve with conservative management alone. The Myocarditis Treatment Trial was designed to address the role of immunosuppression in the treatment of myocarditis, define the natural history of the disease, and increase understanding of the immunologic mechanisms involved in the pathogenesis of the disease. Enrollment began in October 1986 and was completed in October 1990 with follow-up completed in 1991. The study failed to show a significant benefit for immunosuppressive agents. Conventional medical regimens should be used to alleviate symptoms and immunosuppressive agents should be reserved for patients with progressive deterioration and biopsy-proven disease.