Oxygen consumption during ambulation was measured in nine middle-aged or elderly subjects having above-knee amputations, in an effort to determine the mode of ambulation and prosthetic prescription requiring the least energy expenditure. Studies revealed that crutch walking and prosthetic ambulation require the same energy expenditure; that there is no significant difference between the energy requirements of ambulating with knee locked or unlocked; and that 65 percent more energy is required at approximately one-half the normal speed of ambulation for above-knee amputees as compared to normal persons.