The recent competitive successes of a bilateral, transtibial amputee sprint runner who races with modern running prostheses has triggered an international controversy regarding the relative function provided by his artificial limbs. Here, we conducted three tests of functional similarity between this amputee sprinter and competitive male runners with intact limbs: the metabolic cost of running, sprinting endurance, and running mechanics. Metabolic and mechanical data, respectively, were acquired via indirect calorimetry and ground reaction force measurement during constant-speed, level treadmill running. First, we found that the mean gross metabolic cost of transport of our amputee sprint subject (174.9 ml O₂ kg^-1 km^-1; speeds: 2.5 to 4.1 m s^-1) was only 3.8% lower than mean values for intact-limb elite distance runners and 6.7% lower than for sub-elite distance runners, but 17% lower than for intact-limb 400-meter specialists (210.6 [13.2; SD] ml O₂ kg^-1 km^-1). Second, the speeds our amputee sprinter maintained for six all-out, constant-speed trials to failure (speeds: 6.6-10.8 m s^-1; durations: 2-90 s) were within 2.2 [0.6]% of those predicted for intact-limb sprinters. Third, at sprinting speeds of 8.0, 9.0 and 10.0 m s^-1, our amputee subject had longer foot-ground contact times (+14.7 [4.2]%), shorter aerial (-26.4 [9.9]%) and swing times (-15.2 [6.9]%), and lower stance-averaged vertical forces (-19.3 [3.1]%) than intact-limb sprinters (top speeds = 10.8 vs.10.8 [0.6] m s^-1). We conclude that running on modern, lower-limb sprinting prostheses appears to be physiologically similar, but mechanically different than running with intact limbs.