Today, most elite endurance athletes use high-altitude training to some extent. For at least the last 40 years, it has been linked to increased performance. But how was high-altitude training established as a means of improving performance? And how did the scientific approach to altitude differ from the traditional, natural valuation of mountains as a site for training? In this essay, these questions are addressed.High-altitude training was introduced in sports in the post-war period. During the 1960s, it became a highly contested method, with controversies between scientists, athletes, doctors, sport organizations and coaches. What ideas about altitude and performance were important in this process? What type of scientific hypotheses led scientists and sport practitioners towards increasing high-altitude training? Interestingly, those within sports who rejected the scientific, ‘machine-like’ training methods also often valued the mountains. Famous Swedish coach Gösta Olander is one example. He was the most influential protagonist of the natural training method in Sweden, and his base was in Vålådalen (in Jämtland, near Östersund and Åre). Both Swedish (e.g. Sixten Jernberg, Gunder Hägg) and international athletes (e.g. Michel Jazy and Michel Bernard) came to Vålådalen. The fresh mountain air and scenic surroundings were important as a place for training camps, but scientists later demystified the mountains via scientific explanations about increased oxygen uptake and increasing hemoglobin levels in the blood. Vålådalen became a center not only for natural training, but also for scientific monitoring, testing and evaluation.And the setting of international standards regarding high-altitude training had a political aspect, as the issue was addressed when white runners from low altitude were threatened by the results of mainly runners from high altitude countries like Kenya and Ethiopia.Focusing on the Swedish case, we will analyze the scientific interest in high-altitude training for sports. Especially, we will study the links between science, military and sports.