The purpose of the present study was to investigate the relationship between aerobic characteristics and sprint performance, and the effects of high-intensity endurance training in cross-country skiing. Ten male and five female elite junior cross-country skiers performed an 8-week intervention training period. The intervention group (IG, n = 7) increased the volume of high-intensity endurance training performed in level terrain, whereas the control group (CG, n = 8) continued their baseline training. Before and after the intervention period, the skiers were tested for 1.5 km time-trial performance on roller skis outdoors in the skating technique. Maximal oxygen uptake (VO₂max) and oxygen uptake at the ventilatory threshold (VO₂VT) were measured during treadmill running. VO₂max and VO₂VT were closely related to sprint performance (r = ~0.75, both p < 0.008). The IG improved sprint performance, VO₂max and VO₂VT from pre to post-testing, and improved sprint performance and VO₂VT when compared to the CG (all p < 0.01). This study shows a close relationship between aerobic power and sprint performance in cross-country skiing, and highlights the positive effects of high-intensity endurance training in level terrain.