With the exception of a few analyses of the impacts of ASHRAE Standard 62-89 and energy use in specific buildings, the energy use in commercial building due to infiltration and ventilation flows has received little attention. However, as improvements have been made in insulation, windows, etc., the relative importance of these airflows has increased. Previous work at NIST described a research plan to quantify, and assess opportunities to reduce, the energy and indoor air quality impacts of building airtightness and ventilation system control in office buildings (Emmerich et al. 1995). It included an initial estimate that infiltration is responsible for 18% of the total heating energy use and 2% of the total cooling energy use in US office buildings but also concluded that an improved estimate would require the development of a new simulation tool coupling a multizone airflow model with a building thermal analysis program. McDowell et al. (2003) describes the incorporation of the AIRNET airflow model (the airflow simulation portion of the CONTAMW multizone IAQ modeling program) into the TRNSYS energy simulation program to be the needed tool. The resulting integrated simulation tool was then used to estimate the energy usage of 25 buildings representing the U.S. office building stock over a range of infiltration and ventilation conditions. This paper presents detailed simulation results including infiltration rates and their associated heating and cooling loads with an emphasis on the results from the buildings representing recent construction.