This is a combined study using in situ hybridization and the polymerase chain reaction to investigate the presence of Coxsackie virus RNA in formalin-fixed tissue from cases of childhood myocarditis. Of the ten cases studied, two were positive by both methods. The virus RNA was predominantly located in areas showing an inflammatory cell infiltrate and myofibre necrosis. These findings suggest that direct lytic infection of myocytes by virus is responsible for myocarditis in these cases, rather than an autoimmune process, which has been suggested previously. The findings in one case, where the virus showed a marked sub-endocardial distribution, may have implications for the aetiology of endocardial fibroelastosis by confirming a viral tropism for this location. The techniques used in this study are easily repeatable and can be directly applied to look for viruses in a number of other diseases where a viral aetiology is suspected.