: The oxygen consumption during walking has been studied in 13 patients recovering from fractures of the leg and in 8 control subjects. Oxygen consumption of normal subjects when walking at velocities of 0.5, 1.0 and 1.5 m sec-1 was 8.0, 9.8 and 13.6 ml kg-1 min-1 respectively. For subjects walking on crutches with the leg encased in a Plaster of Paris cast, the oxygen consumption at these velocities was 12.0, 17.5 and 25.3 ml kg-1 min-1 respectively. The energy cost of walking was reduced when the cast was removed, when the crutches were replaced by sticks and when all aides were discarded. When all aids had been discarded the energy expenditure was normal at 0.5 and 1.0 m sec-1 but was still significantly higher at faster velocities.