Hematological and physiological variations were assessed during an altitude training camp in professional cyclists, with precise training load monitoring. External to internal training load ratio defined as power output (W) per heartbeat (bpm) (W/HR) was investigated as a novel metric pertaining altitude acclimatization and training camp response. Nineteen male professional cyclists participated in a 3-week Live-High Train-Low (LHTL) camp. Blood samples were collected to assess hemoglobin concentration ([Hb]), hematocrit (HCT), reticulocyte percentage (RET%), and immature reticulocyte fraction (IRF). Total hemoglobin mass (Hbmass) and plasma volume (PV) were measured upon arrival (pre-) and on the last day (post-) at altitude. Training load was monitored across the camp with power output (W), duration (min), heart rate (HR, bpm), and total work (W; kJ) recorded. W/HR was computed from each session’s average W and HR. From Pre- to Post-, Hbmass increased by 3.7% (p < 0.001), while PV decreased by 4.4% (p < 0.05). [Hb], HCT, RET%, and IRF were elevated (+4.1%; +6.2%; +31.0%; and +50.0%; respectively). Training load increased progressively from the lead-in phase to the second week (+68%; +83%; respectively). W/HR decreased by 11% (p < 0.01) at the onset of the hypoxic exposure before improving gradually again. W/HR in normoxia was 7% higher post-camp compared to the lead in phase (p < 0.01). Overall, the altitude training elicited positive hematological adaptations in professional cyclists. The increase in W/HR after the camp could suggest an enhanced cardiovascular efficiency and/or peripheral oxygen utilization. W/HR may represent a pertinent metric beyond Hbmass with its concomitant response to altitude and training stimuli.