It is well established that the energy cost per unit distance traveled is minimal at an intermediate walking speed in humans, defining an energetically optimal walking speed. However, little is known about the optimal walking speed while carrying a load. In this work, we studied the effect of speed and load on the energy expenditure of walking. The O(2) consumption and CO(2) production were measured in ten subjects while standing or walking at different speeds from 0.5 to 1.7 m s(-1) with loads from 0 to 75% of their body mass (M(b)). The loads were carried in typical trekker's backpacks with hip support. Our results show that the mass-specific gross metabolic power increases curvilinearly with speed and is directly proportional to the load at any speed. For all loading conditions, the gross metabolic energy cost (J kg(-1) m(-1)) presents a U-shaped curve with a minimum at around 1.3 m s(-1). At that optimal speed, a load up to 1/4 M(b) seems appropriate for long-distance walks. In addition, the optimal speed for net cost minimization is around 1.06 m s(-1) and is independent of load.