Background: Studies suggest that the energy expenditure of healthy persons (control) during walking with the preferred walking speed in steady-state conditions is dominated by fat oxidation. Conversely, carbohydrate and fat oxidation during walking is little investigated in transfemoral amputees. Objectives: To investigate carbohydrate and fat oxidation, energy cost of walking, and percent utilization of maximal aerobic capacity (%V·O2max) during walking. Study design: Eight transfemoral amputees and controls walked with their preferred walking speed and speeds 12.5% and 25% slower and faster than their preferred walking speed. Methods: Energy expenditure and fuel utilization were measured using a portable metabolic analyzer. Metabolic values are means ± standard deviation. Results: For transfemoral amputees (37.0 ± 10.9 years) and controls (39.0 ± 12.3 years), fat utilization at the preferred walking speed was 44.8% ± 7.2% and 45.0% ± 7.2% of the total energy expenditure, respectively. The preferred walking speed of the transfemoral amputees and controls was close to a metabolic cross-over speed, which is the speed where carbohydrate utilization increases steeply and fat utilization decreases. When walking fast, at 90 m min−1 (preferred walking speed plus 25%), transfemoral amputees utilized 70.7% ± 5.6% of their V·O2max , while the controls utilized 30.9% ± 4.5% (p < 0.001) at the matching speed (control preferred walking speed). At 90 m min−1, carbohydrate utilization was 78% ± 4.7% and 55.2% ± 7.2% of the total energy expenditure for the transfemoral amputees and controls, respectively (p < 0.01). Compared to the control, energy cost of walking was higher for the transfemoral amputees at all speeds (all comparisons; p < 0.001). Conclusion: At the preferred walking speed, carbohydrate, not fat, dominates energy expenditure of both transfemoral amputees and controls. For the transfemoral amputees, consequences of fast walking are very high V·O2max utilization and rate of carbohydrate oxidation. Clinical relevance Research on the relationships between physical effort and fuel partitioning during ambulation could provide important insights for exercise-rehabilitation programs for lower limb amputees (LLA). Regular endurance exercise will improve maximal aerobic capacity and enable LLA to walk faster and at the same time expend less energy and improve fat utilization.