Enteroviruses are potential etiologic agents of myocarditis and dilated cardiomyopathy (DCM). A recently developed molecular approach has offered evidence of viral infection by detecting the virus genome. The nested reverse transcriptase polymerase chain reaction (nRT-PCR) was used to detect enteroviral RNA in endomyocardial biopsy tissues of myocarditis and DCM. The authors examined 44 tissues obtained from 36 patients with myocarditis, as well as from 10 patients with non-infectious cardiac diseases as controls. Enteroviral RNA was detected in 12 of 36 patients with myocarditis. The second endomyocardial biopsy was carried out in five of the patients, in whom enteroviral RNA was detected at the first biopsy, at intervals from 3 weeks to 8 years after the first biopsy, and enteroviral RNA was found in four and had disappeared in one. In one of the four positive patients at the second biopsy, a third biopsy was carried out 5 months later (6 months after the first), and the RNA was detected. Active myocarditis became clinically and microscopically mild at the second and third biopsies. In one patient who developed DCM, enteroviral RNA was also detected at a second biopsy performed 8 years after the first. Enteroviral infection is a probable cause of myocarditis and enterovirus-infected myocarditis may progress to DCM.