Energetic metabolism during effort is impaired in patients with left ventricular dysfunction (DYSF), but data have been lacking up to now on the relative anaerobic vs aerobic contribution to total energy release during supramaximal effort. Recently, the maximal accumulated oxygen deficit (MAOD) has been shown to be measurable in DYSF patients, making it possible to evaluate the anaerobic/aerobic interaction under conditions of maximal stress of both anaerobic and aerobic metabolic pathways in this population. Nineteen DYSF patients and 17 normals (N) underwent 1 ramp cardiopulmonary, 3 moderate-intensity constant-power, and 3 supramaximal constant-power (1-2min, 2-3min, and 3-4min duration) exercise tests. MAOD was the difference between accumulated O₂ demand (accO₂dem - estimated from the moderate-intensity VO2/Watt relationship) and uptake during supramaximal tests. Percent anaerobic (%ANAER) and aerobic (%AER) energetic release were [(MAOD/ accO₂dem) * 100] and 100-%ANAER, respectively. MAOD did not vary between 1-2min, 2-3min, and 3-4min supramaximal tests, whereas accO2dem increased significantly with and was linearly related to test duration in both DYSF and N. Consequently, %ANAER and %AER decreased and increased, respectively, with increasing test duration, but did not differ between DYSF and N in 1-2min, 2-3min, and 3-4min tests. Our study demonstrates a similar relative anaerobic vs aerobic contribution to total energy release during supramaximal effort in DYSF and N. This finding indicates that energetic metabolism during supramaximal exercise is exercise tolerance-independent, and that relative anaerobic vs aerobic contribution in this effort domain remains the same within the physiology- or pathology-induced limits to individual peak exercise performance.