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%, P < 0.001) and in NH (918–953 g, 3.8 ± 2.6%, P < 0.001). Hb_mass changes did not differ between HH and NH ( P = 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 ( r = 0.52, P = 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. NEW & NOTEWORTHY 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.