High-intensity interval training (HIIT; alternating short, intense bouts of exercise with short rest periods) is known to accelerate fitness improvements in healthy populations. This could be an effective strategy for those with asthma, as rest periods are introduced before an asthma attack is likely to begin. This is particularly important for youth, for whom asthma is the most common respiratory disease. In this study, 35 high school adolescents (17 with asthma) took part in three, 30-minute HIIT sessions per week for six months. Before, during, and after the six-months, their fitness levels were compared to 34 other children (19 with asthma) who hadn’t been asked to perform the HIIT sessions. Aerobic fitness markers improved in the HIIT group, with the presence of asthma having no effect on the improvement. HIIT led to the body being able to respond to a quick increase in exercise demand, possibly due to the working muscles getting faster at using the oxygen delivered to them. These results indicate that HIIT can improve the fitness of adolescents, even if they have asthma.