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Elite athletes often undertake multiple altitude exposures within and between training years in an attempt to improve sea level performance.To quantify the reproducibility of responses to live high/train low (LHTL) altitude exposure in the same group of athletes.Sixteen highly trained runners with maximal aerobic power (V˙O2max) of 73.1 ± 4.6 and 64.4 ± 3.2 mL·kg−1·min−1 (mean ± SD) for males and females, respectively, completed 2 × 3-wk blocks of simulated LHTL (14 h·d−1, 3000 m) or resided near sea level (600 m) in a controlled study design. Changes in the 4.5-km time trial performance and physiological measures including V˙O2max, running economy and hemoglobin mass (Hbmass) were assessed.Time trial performance showed small and variable changes after each 3-wk altitude block in both the LHTL (mean [±90% confidence limits]: −1.4% [±1.1%] and 0.7% [±1.3%]) and the control (0.5% [±1.5%] and −0.7% [±0.8%]) groups. The LHTL group demonstrated reproducible improvements in V˙O2max (2.1% [±2.1%] and 2.1% [±3.9%]) and Hbmass (2.8% [±2.1%] and 2.7% [±1.8%]) after each 3-wk block. Compared with those in the control group, the runners in the LHTL group were substantially faster after the first 3-wk block (LHTL − control = −1.9% [±1.8%]) and had substantially higher Hbmass after the second 3-wk block (4.2% [±2.1%]). There was no substantial difference in the change in mean V˙O2max between the groups after the first (1.2% [±3.3%]) or second 3-wk block (1.4% [±4.6%]).Three-week LHTL altitude exposure can induce reproducible mean improvements in V˙O2max and Hbmass in highly trained runners, but changes in time trial performance seem to be more variable. Competitive performance is dependent not only on improvements in physiological capacities that underpin performance but also on a complex interaction of many factors including fitness, fatigue, and motivation.