The purpose of this research was to compare individual hemoglobin mass (Hb _mass ) changes following a live high-train low (LHTL) altitude training camp under either normobaric hypoxia (NH) or hypobaric hypoxia (HH) conditions in endurance athletes. In a crossover design with a one-year washout, 15 male triathletes randomly performed two 18-day LHTL training camps in either HH or NH. All athletes slept at 2,250 meters and trained at altitudes <1,200 meters. Hb _mass was measured in duplicate with the optimized carbon monoxide rebreathing method before (pre) and immediately after (post) each 18-day training camp. Hb _mass increased similarly in HH (916-957 g, 4.5 ± 2.2%, <i>P</i> < 0.001) and in NH (918-953 g, 3.8 ± 2.6%, <i>P</i> < 0.001). Hb _mass changes did not differ between HH and NH ( <i>P</i> = 0.42). There was substantial interindividual variability among subjects to both interventions (i.e., individual responsiveness or the individual variation in the response to an intervention free of technical noise): 0.9% in HH and 1.7% in NH. However, a correlation between intraindividual ΔHb _mass changes (%) in HH and in NH ( <i>r</i> = 0.52, <i>P</i> = 0.048) was observed. HH and NH evoked similar mean Hb _mass increases following LHTL. Among the mean Hb _mass changes, there was a notable variation in individual Hb _mass response that tended to be reproducible. <b>NEW & NOTEWORTHY</b> This is the first study to compare individual hemoglobin mass (Hb _mass ) response to normobaric and hypobaric live high-train low using a same-subject crossover design. The main findings indicate that hypobaric and normobaric hypoxia evoked a similar mean increase in Hb _mass following 18 days of live high-train low. Notable variability and reproducibility in individual Hb _mass responses between athletes was observed, indicating the importance of evaluating individual Hb _mass response to altitude training.