This double-blind, randomized, placebo controlled trial examined the effects of 4 wk of resting exposure to intermittent hypobaric hypoxia (IHE, 3h/d, 5d/wk at 4,000-5,500 m) or normoxia combined with training at sea level on performance and maximal oxygen transport in athletes. 23 trained swimmers and runners completed duplicate baseline time trials (100/400-m swims, or 3-km run) and measures for maximal VO₂, V_E, heart rate (HRmax), and VO₂ at the ventilatory threshold (VT) during incremental treadmill or swimming flume tests. Subjects were matched for gender, sport, performance and training status, and divided randomly between hypobaric hypoxia (HYPO, n = 11) and normobaric normoxia (NORM, n = 12) groups. All tests were repeated within the first (POST1) and third weeks (POST2) after the intervention. Time-trial performance did not improve in either group. We could not detect a significant difference between groups for a change in VO₂max, V_Emax, HRmax, or VO₂ at VT after the intervention (group x test interaction p = 0.31, 0.24, 0.26, 0.12 respectively). When runners and swimmers were considered separately, HYPO swimmers appeared to increase VO₂max (+6.2 %, interaction p = 0.07) at POST2 following a pre-competition taper, and increased VO₂ at VT (+8.9 and +12.1 %, interaction p = 0.007 and 0.006, at POST1 and POST2). We conclude that this "dose" of IHE was not sufficient to improve performance or oxygen transport in this heterogeneous group of athletes. Whether there are potential benefits of this regimen for specific sports or training/tapering strategies may require further study.