The single-breath transfer factor (TLCO) was measured in ten healthy subjects at sea level and at a simulated altitude of 3,048 m. Measurements were made at sea level using 21% oxygen and at altitude using both 21% and 31% oxygen in the test gas. At 3,048 m altitude (PB = 69.7 kPa), the mean TLCO increased 14% or about 2.3% per kPa (0.3% per mmHg) decrease in inspired oxygen pressure. Our data also suggests that altering the test gas oxygen concentration to account for the change in PAO2 due to altitude results in a TLCO equivalent to that obtained at sea level.