To explore the role of peroxisome proliferator-activated receptor α (PPARα)-mediated derangements in myocardial metabolism in the pathogenesis of diabetic cardiomyopathy, insulinopenic mice with PPARα deficiency (PPARα^−/−) or cardiac-restricted overexpression [myosin heavy chain (MHC)-PPAR] were characterized. Whereas PPARα^−/− mice were protected from the development of diabetes-induced cardiac hypertrophy, the combination of diabetes and the MHC-PPAR genotype resulted in a more severe cardiomyopathic phenotype than either did alone. Cardiomyopathy in diabetic MHC-PPAR mice was accompanied by myocardial long-chain triglyceride accumulation. The cardiomyopathic phenotype was exacerbated in MHC-PPAR mice fed a diet enriched in triglyceride containing long-chain fatty acid, an effect that was reversed by discontinuing the high-fat diet and absent in mice given a medium-chain triglyceride-enriched diet. Reactive oxygen intermediates were identified as candidate mediators of cardiomyopathic effects in MHC-PPAR mice. These results link dysregulation of the PPARα gene regulatory pathway to cardiac dysfunction in the diabetic and provide a rationale for serum lipid-lowering strategies in the treatment of diabetic cardiomyopathy.