Purpose The aim of this study is to describe the effects of speed and gradient during uphill ski mountaineering on energy expenditure, to relate any changes to changes in stride characteristics, and to determine an optimal gradient and speed allowing minimization of energy expenditure. Methods 11 subjects were tested on snowy trails using their mountaineering skis (fitted with skins), boots and poles, at three gradients (7, 11 and 33 %) at 80 % of maximum heart rate (HRmax), and at 11 % also at 90 and 100 % of HRmax. Energy expenditure was calculated by indirect calorimetry to derive energy cost of locomotion (EC), vertical energy cost (ECvert) and mechanical efficiency, while stride length, stride frequency, relative and absolute thrust phase duration, and slope gradient were measured with an inertial sensor-based system. Results At 11 % there was no change with speed in EC, ECvert and mechanical efficiency, while stride length and frequency increased and absolute thrust phase duration decreased. There was an effect of gradient on EC, ECvert and mechanical efficiency, while speed, stride length and stride frequency decreased and absolute and relative thrust phase duration increased. The most economical gradient (lowest ECvert) was the steepest one. Conclusion During ski mountaineering uphill at shallow gradient (11 %), EC, ECvert and mechanical efficiency do not vary with speed, while at steeper gradient (33 %) economy is improved. It follows that to minimize energy expenditure and optimize performance to reach a place located at a higher altitude, an athlete should choose a steep gradient, if he/she is able to maintain a sufficient speed.