Accurate differentiation of active inflammatory diseases of the myocardium from idiopathic and chronic forms of dilated cardiomyopathy is clinically important because their long-term outcomes differ. Accurately diagnosing myocardial histopathology may be more relevant in the near future as effective treatments for myocarditis, sarcoid, and other myocardial diseases evolve. The cost and the low sensitivity and specificity for endomyocardial biopsy suggest that noninvasive testing is essential for deciding which patients with unexplained cardiomyopathy should undergo this procedure. The early identification of patients with active myocarditis may more precisely reformulate the approach to the diagnosis and treatment of unexplained cardiomyopathy.