Adipose substitution of ventricular myocardium is characteristic of arrhythmogenic right ventricular cardiomyopathy, but is also found in other heart conditions. It is thought to be a consequence of myocyte loss due to myocarditis or other noxious stimuli. We describe a unique case of cardiomyopathy with a morphologic pattern suggestive of transdifferentiation from myocytes to mature adipocytes. Gross, histologic, and ultrastructural examination were performed on the heart of a female transplant patient with a clinical diagnosis of familial dilated cardiomyopathy. Gross examination showed fibroadipose substitution of the left ventricle and adipose replacement of the right. Histology, immunohistochemistry, and ultrastructure were highly suggestive of transdifferentiation from cardiac muscle to adipose tissue. Myocyte transdifferentiation could represent an alternative pathogenetic pathway to the myocyte-loss and adipose-replacement mechanism in arrhythmogenic right ventricular cardiomyopathy, or it could be the basis of a new type of familial cardiomyopathy.