This study tested the effects of "living high-training low" (Hi-Lo) on aerobic performance and economy of work in elite athletes. Forty endurance athletes (cross-country skiers, swimmers, runners) performed 13-18 consecutive days of training at 1,200 m altitude, by sleeping at 1,200 m (LL, n = 20) or in hypoxic rooms with 5-6 nights at 2,500 m followed by 8-12 nights at 3,000-3,500 m (HL, n = 20). The athletes were evaluated before (pre-), one (post-1) and 15 days (post-15) after Hi-Lo. Economy was assessed from two sub-maximal tests, one non-specific (cycling) and one specific (running or swimming). From pre- to post-1: V(O2)max increased both in HL (+ 7.8%, P < 0.01) and in LL (+ 3.3%, P < 0.05), peak power output (PPO) tended to increase more (P=0.06) in HL (+ 4.1%, P < 0.01) than in LL (+ 1.9%). At post-15, V(O2)max has returned to pre-values in both groups, PPO increased more (P < 0.05) in HL (+ 8.3%, P < 0.01) than in LL (+ 3.8%), V(O2) and power at respiratory compensation point (RCP) increased more (P < 0.05) in HL (+ 9.5%, P < 0.01 and + 11.2%, P < 0.01) than in LL (+ 3.2 and + 3.3%). Cycling mechanical efficiency (8-5%) and economy during specific locomotion (7-7%) increased (P < 0.05) in both groups. This study shows that, for a similar increase in V(O2)max HL had a greater increase in PPO than LL. The efficiency of Hi-Lo is also evidenced 15 days later by higher V(O2) and power at RCP. This study emphasizes that during the post-altitude period, economy of work greatly increases in both groups.