We evaluated the efficacy of heart rate variability (HRV)-guided training in adolescent athletes during a 2-week, high altitude ( approximately 1900 m) training camp. Sixteen middle- and long-distance runners (4 female/12 male, 16.9 +/- 1.0 years, 65.44 +/- 4.03 mL.kg(-1).min(-1)) were divided into 2 matched groups, both of which received the same training plan, but one of which acquired postwaking HRV values that were used to tailor the training prescription. During the camp, seven athletes in the HRV-guided group combined for a total of 32 training adjustments, whereas there were only 3 runners combined for 14 total training adjustments in the control group. A significant group by time interaction (p < 0.001) for VO(2)max was driven by VO(2)max improvements in the HRV group (+2.8 mL.kg(-1).min(-1), +4.27%; p(Bonf) = 0.002) that were not observed in the control condition (+0.8 mL.kg(-1).min(-1), +1.26%; p(Bonf) = 0.643). After returning from the camp, all athletes in the HRV group set a personal best, and six out of eight achieved their best positions in the National Championship, whereas only 75% of athletes in the control group set a personal best and five out of eight achieved their best positions in the National Championship. These data provide evidence in support of HRV-guided training as a way to optimize training prescriptions in adolescent athletes.