We sought to determine whether improved cycling performance following ‘Live High-Train Low’ (LHTL) occurs if increases in haemoglobin mass (Hb mass ) are prevented via periodic phlebotomy during hypoxic exposure. Eleven, highly trained, female cyclists completed 26 nights of simulated LHTL (16 h day −1 , 3000 m). Hb mass was determined in quadruplicate before LHTL and in duplicate weekly thereafter. After 14 nights, cyclists were pair-matched, based on their Hb mass response (ΔHb mass ) from baseline, to form a response group (Response, n  = 5) in which Hb mass was free to adapt, and a Clamp group (Clamp, n  = 6) in which ΔHb mass was negated via weekly phlebotomy. All cyclists were blinded to the blood volume removed. Cycling performance was assessed in duplicate before and after LHTL using a maximal 4-min effort (MMP 4min ) followed by a ride time to exhaustion test at peak power output ( T lim ). VO 2peak was established during the MMP 4min . Following LHTL, Hb mass increased in Response (mean ± SD, 5.5 ± 2.9%). Due to repeated phlebotomy, there was no ΔHb mass in Clamp (−0.4 ± 0.6%). VO 2peak increased in Response (3.5 ± 2.3%) but not in Clamp (0.3 ± 2.6%). MMP 4min improved in both the groups (Response 4.5 ± 1.1%, Clamp 3.6 ± 1.4%) and was not different between groups ( p  = 0.58). T lim increased only in Response, with Clamp substantially worse than Response (−37.6%; 90% CL −58.9 to −5.0, p  = 0.07). Our novel findings, showing an ~4% increase in MMP 4min despite blocking an ~5% increase in Hb mass , suggest that accelerated erythropoiesis is not the sole mechanism by which LHTL improves performance. However, increases in Hb mass appear to influence the aerobic contribution to high-intensity exercise which may be important for subsequent high-intensity efforts.