Fifty-two patients with recently diagnosed idiopathic dilated cardiomyopathy were studied to determine the incidence of myocarditis; patients were randomly assigned to receive either conventional therapy alone or conventional therapy plus prednisone to assess possible therapeutic efficacy with regard to survival. Inflammatory criteria were present in 23% of the population studied with 13% having overt myocarditis according to the Dallas criteria. The addition of prednisone to conventional therapy did not improve survival in a homogeneous population with new-onset dilated cardiomyopathy. Furthermore, the diagnosis of myocarditis by endomyocardial biopsy did not influence 2-year survival once dilated cardiomyopathy had developed. Biopsy-documented myocarditis resolved in all patients, according to results of 3-month follow-up endomyocardial biopsies, regardless of treatment group. There was a trend for patients with a left ventricular ejection fraction less than 20% to show reduced survival at 2 years compared to the group with a higher ejection fraction (p = 0.07). Right ventricular dysfunction determined at catheterization was present in 20 of 52 patients and was the most significant predictor of survival. Patients with preserved right ventricular function had a 95% 24-month survival rate compared to 47% for patients with right ventricular diastolic dysfunction (right ventricular end-diastolic pressure greater than or equal to 11 mm Hg) (p = 0.005).