INTRODUCTION: Reaching an international level in any endurance sport requires a large volume of systematic training performed over time. While the annual training characteristics of senior, elite-level cross-country (XC) skiers are well documented (1), limited data exist regarding the long-term training of developing XC skiers. The current study aimed to describe the training undertaken by a group of highly-trained XC skiers throughout their transition from junior- to senior-level athletes. METHODS: In this retrospective cohort study, self-reported training data were obtained from 32 highly-trained female (n = 13) and male (n = 19) XC skiers for the season they turned 16 years old (y) until the season they turned 23 y. At the time of inclusion, 26 skiers (11 females and 15 males) had represented at least one of the Swedish national teams (senior, development or junior), and eight of these skiers (6 females and 2 males) had won at least one individual U23 or Junior World Championship medal. The remaining six skiers were part of a specialist ski university in Sweden, where selection is based on the potential to perform at a world-class level. Training data were organized by training form (endurance, strength, and speed), mode (e.g. on-snow skiing, roller skiing, running, and cycling), and intensity (using a 4-zone model), which were then divided into five annual training phases (transition, general preparation [GP], specific preparation [SP], competition [CP], regeneration). RESULTS: Data from 155 seasons, including 59 026 individual training sessions and 94 964 h of training, were analysed. From age 16 to 22 y the total volume of endurance training increased from 472 ± 70 to 721 ± 86 h/yr (p < 0.001). Low-intensity training (LIT, below the first lactate threshold, <85% HRmax) and high-intensity training (HIT, above the first lactate threshold, >85% HRmax) increased from 414 ± 61 to 656 ± 72 h/yr (p < 0.001) and 58 ± 33 to 65 ± 16 h/yr (p = 0.018), respectively. The training-volume distribution developed progressively from a more even distribution across training phases at age 16 y (GP: 10.6 ± 1.8 h/wk; SP: 10.4 ± 1.5 h/wk; CP: 8.6 ± 1.5 h/wk) to a more traditional periodised model at age 22 y (GP: 17.5 ± 1.7 h/wk; SP: 12.7 ± 1.9 h/wk; CP: 11.1 ± 2.1 h/wk), whereby a higher proportion of the total training volume was performed in GP, and a lower proportion in SP and CP, as athletes developed. CONCLUSION: In conclusion, this group of highly-trained XC skiers progressively increased their endurance training volume from age 16 to 22 y, to a level that is required of elite XC skiers. This increase in training volume was primarily due to an increase in LIT in the general preparation phase. In addition, training-volume distribution became more periodised as athletes developed from junior to senior level. REFERENCES 1. Ø. Sandbakk & HC. Holmberg, International Journal of Sports Physiology and Performance 9, 117-121 (2014).