Aeromedical transport of critically ill patients requires continued, accurate performance of equipment at altitude. Changes in barometric pressure can affect the performance of mechanical ventilators calibrated for operation at sea level. Deploying ventilators that can maintain a consistent tidal volume (VT) delivery at various altitudes is imperative for lung protection when transporting wounded warfighters to each echelon of care. Three ventilators (Impact 731, Hamilton T1, CareFusion ReVel) were tested at pediatric (50 & 100 mL) and adult VT (250-750 mL) at 0 and 20 cm H2O positive end expiratory pressure and at inspired oxygen of 0.21 and 1.0. Airway pressure, volume, and flow were measured at sea level, 8,000, 16,000, and 22,000 feet (corresponding to barometric pressures of 760, 564, 412, and 321 mmHg) using a calibrated pneumotachograph connected to a training test lung in an altitude chamber. Set VT and delivered VT and changes in VT at each altitude were compared by t-test. Only the T1 delivered VT within 10% of set VT at 8,000 feet. Mean VT was less than set VT at sea level as a result of circuit compressible volume with the ReVel and 731. Changes in VT varied widely among the devices at sea level and at altitude. Increasing altitudes resulted in larger VT than set for the ReVel and T1. The 731 delivered VT within 10% at the adult settings at all altitudes. Altitude compensation is an active software algorithm. Only the 731 actively accounts for changes in barometric pressure to maintain the set VT at all tested altitudes.