The energy costs of skiing with the diagonal stride technique was investigated in 15 members of the men's and women's national cross-country ski teams and in 5 recreational skiers. Oxygen uptake was measured directly by means of a portable bag-method for each subject during level skiing at 3 different skiing speeds. The oxygen cost of double poling vs. diagonal stride techniques was also measured in 3 subjects during level skiing. Although a strong linear relationship was found between VO2 and speed of skiing (r = 0.868, p less than 0.001), there was considerable variation even among elite competitors. The VO2-skiing speed relationship was parallel to that obtained for running, but the O2 cost of skiing was found to be 10--12 ml/kg in excess of that predicted for level running at the same speed. On flat terrain, the O2 cost of the double poling technique as used in fast skiing was found to be higher than that for diagonal striding.