In rare cases, the diagnosis of hypertrophic and dilated cardiomyopathy (DCM) in children was established postmortem. Our case report deals with the sudden and unexpected death of an 8-year-old boy. The postmortem examination revealed non-obstructive hypertrophy with irregular arrangement of muscular fibers, dilatation of the ventricles, endocardial fibrosis, microfocal vacuolization with enlarged hyperchromatic nuclei, and signs of inflammation with interstitial fibrosis. We present an evolution from idiopathic cardiomyopathy to DCM. To some extent, there were morphologic signs of an inflammatory process that first led us to suspect a specific inflammatory DCM.