This study had two goals: 1) measure hematologic changes with high-altitude acclimatization in horses; and 2) assess the effect of 9 days at high altitude on subsequent athletic performance at low altitude. Six horses performed standardized exercise tests on a dirt track (before and during time at altitude) and treadmill (pre- and postaltitude exposure). Resting and immediate postexercise blood samples were measured for blood volume, lactate, red cell number, packed cell volume, and 2,3-diphosphoglycerate (DPG) concentrations at 225 m, over a 9-day period at 3,800 m, and shortly after returning to 225 m. Acclimatization produced increases in total red cell volume (38.2 ± 2.4 to 48.1 ± 2.9 ml/kg, P = 0.004) and DPG/hemoglobin concentrations (19.4 ± 1.7 increased to 29.4 ± 0.4 µmol/g, P = 0.004). Two performance variables, heart rate recovery postexercise and lactate recovery, were faster after acclimatization.