Heart failure (HF) with left ventricular systolic dysfunction (LVSD) is associated with a shift in substrate utilization and a compromised energetic state. Whether these changes are connected with mitochondrial dysfunction is not known. We hypothesized that the cardiac phenotype in LVSD could be caused by reduced mitochondrial oxidative phosphorylation (OXPHOS) capacity and reduced mitochondrial creatine kinase (miCK) capacity. The study aim was to test mitochondrial OXPHOS capacity in LVSD myocardium compared with OXPHOS capacity in a comparable patient group without LVSD.