Underfloor air distribution (UFAD) is a method of supplying conditioned air via floor mounted air supply diffusers. We measured several aspects of the performance of an UFAD system in an office building. The air change effectiveness was very close to unity, which is comparable to that measured in buildings with typical overhead air distribution. The pollutant removal efficiency for carbon dioxide was 13% higher than expected with well-mixed air. The increase in air temperatures with height above the floor was only 1 to 2 °C. This amount of thermal stratification could reduce the sensible energy requirements for cooling of outdoor air by approximately 10%. The occupant's satisfaction with thermal conditions was well above average. Thus, the results of this study provide some evidence of moderate energy and IAQ-related benefits of UFAD; however, the benefits need to be confirmed in other studies.