Coxsackievirus B3 is a possible etiologic agent in some forms of myocarditis and idiopathic dilated cardiomyopathy. A method for the detection of coxsackievirus B3 RNA was developed using the polymerase chain reaction based on the amplification of a cDNA copy of the positive-strand viral RNA. The fidelity of the method was established in two murine models for coxsackie B3 myocarditis. All cardiac specimens with adequate RNA for study from coxsackie B3-infected mice contained detectable viral RNA, in contrast to none of control specimens from noninfected mice. The sensitivity of the technique was established at approximately 1 to 100 plaque-forming units of virus per gram of tissue, and the specificity was established as limited to the coxsackievirus B3 serotype among nine viruses tested. In patients with myocarditis, one of five specimens contained detectable viral RNA, whereas none of 11 specimens from patients with idiopathic dilated cardiomyopathy or 21 myocardial specimens from patients with a wide variety of other cardiac disorders contained detectable coxsackie B3 viral RNA. The results show that the polymerase chain reaction is a useful means for detecting coxsackie viral RNA and its application should help in the evaluation of hypotheses concerning the infectious etiology of human myocarditis and idiopathic dilated cardiomyopathy.