EFE, once a common disease of young children, often resulted in congestive heart failure and death. During the past two decades a significant decline in the incidence of EFE occurred. Virus-induced myocarditis had been suspected as the first step in the pathogenesis of the disease; enteroviruses and mumps virus were considered potential etiological agents. Direct evidence for their involvement was limited, however. Recently we demonstrated, by polymerase chain reaction (PCR), that adenoviral DNA sequences could be detected in the myocardium of about 30% of patients with myocarditis; enteroviral sequences were also commonly detected. In this study, myocardial samples from 29 patients with EFE were analyzed for viral genome (i.e., adenovirus, enterovirus, CMV, parvovirus, and mumps) using PCR or reverse transcriptase-PCR. Only adenovirus genome (8/29: 28%) and mumps genome (21/29: 72%) were amplified in these patients samples. In contrast, only 1 of 65 control samples (1.5%) was positive for any of the viruses tested(enterovirus). Two different regions of mumps virus were amplified, the nucleocapsid gene and the polymerase-associated protein gene. Interestingly only 3 of the 21 samples positive for mumps RNA were positive with both sets of primers, indicating that the persistence of mumps virus in the myocardium may be related to the selection of defective virus mutants. These data provide strong evidence for a viral etiology of EFE, supporting the hypothesis that it is a sequela of a viral myocarditis, particularly due to mumps virus. Eradication of mumps virus using vaccination is probably the cause of the decline in the incidence of EFE.