Previous studies have investigated the oxygen cost (0₂) of increasing boot weight during unloaded walking or running, and have shown that for each 100 g increase in weight of footwear there is a 0·7-1·0% increase in O₂ In reality (except in athletic events) the use of heavy footwear is associated with load carriage, usually backpacking. We therefore investigated the effects of increasing boot weight by 5% of body weight on the 0₂ of backpacking a load amounting to 35% of the body-weight in five healthy young males who walked at 4·5 km/hour (0% grade) on a motor-driven treadmill. The results indicated a mean increase of 0·96% in 0₂ whilst backpacking for each 100thinsp;-g increase in boot weight. In contrast the oxygen cost of increasing the backpack load was only 0·15% indicating that it was 6·4 times more expensive to carry weight on the feet as compared to the back. It is concluded that the relation between boot weight and oxygen cost, previously developed for unloaded walking and running, can reasonably be extended to include heavier boots and backpacking.