Building ventilation is a primary determinant of indoor air quality (IAQ) in buildings as it impacts indoor contaminant concentrations and occupant comfort. However, relatively few measurements of office building ventilation performance have been conducted, and those data that exist generally have not employed consistent measurement methods and have not involved representative collections of buildings. The EPA Building Assessment Survey and Evaluation (BASE) study was conducted to assess IAQ, including ventilation, in a large number of randomly selected office buildings throughout the U.S. using a standardized protocol. Ventilation performance assessment was a key part of the survey, and this report presents an analysis of the ventilation data collected during the study. The building evaluations were conducted by EPA contractors, and the analysis presented in this report is based on the ventilation data collected by the contractors.