['Obesity and diabetes mellitus are independently associated with the development of heart failure. In this study, we determined the respective effects of obesity, insulin resistance, and diabetes mellitus on the intrinsic contraction and mitochondrial function of the human myocardium before the onset of cardiomyopathy.', 'Right atrial myocardium was obtained from 141 consecutive patients presenting no sign of cardiomyopathy. We investigated ex vivo isometric contraction, mitochondrial respiration and calcium retention capacity, and respiratory chain complex activities and oxidative stress status. Diabetes mellitus was associated with a pronounced impairment of intrinsic contraction, mitochondrial dysfunction, and increased myocardial oxidative stress, regardless of weight status. In contrast, obesity was associated with less pronounced contractile dysfunction without any significant perturbation of mitochondrial function or oxidative stress status. Tested as continuous variables, glycated hemoglobin A1C, but neither body mass index nor the insulin resistance index (homeostasis model assessment-insulin resistance), was independently associated with cardiac mitochondrial function. Furthermore, diabetes mellitus was associated with cardiac mitochondrial network fragmentation and significantly decreased expression of the mitochondrial fusion related protein MFN1. Myocardial MFN1 content was inversely proportional to hemoglobin A1C.'] Is myocardial contractile dysfunction associated with impaired mitochondrial function and dynamics in type 2 diabetic but not in obese patients?