This double-blind, randomized, placebo-controlled trial examined the effects of 4 wk of resting exposure to intermittent hypobaric hypoxia (IHE, 3 h/day, 5 days/wk at 4,000-5,500 m) or normoxia combined with training at sea level on performance and maximal oxygen transport in athletes. Twenty-three trained swimmers and runners completed duplicate baseline time trials (100/400-m swims, or 3-km run) and measures for maximal oxygen uptake (V̇O 2max), ventilation (V̇Emax), and heart rate (HR max) and the oxygen uptake at the ventilatory threshold (V̇O 2 at VT) during incremental treadmill or swimming flume tests. Subjects were matched for sex, 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 V̇O2max, V̇Emax, HR max, or V̇O2 at VT after the intervention (group X test interaction P = 0.31, 0.24, 0.26, and 0.12, respectively). When runners and swimmers were considered separately, Hypo swimmers appeared to increase V̇O2max (+6.2%, interaction P = 0.07) at Post2 following a precompetition taper and increased V̇O2 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.