The US Green Building Council's (USGBC) LEED guidelines have become the dominant third-party certification program for “green” buildings in the US. Given that buildings use 37% of all energy and 68% of all electricity while contributing substantially to air emission, waste generation, and water consumption issues in the US, one of LEED's purposes is to address the environmental impacts of energy use in buildings. This research analyzes (1) how well the LEED guidelines measure these impacts and (2) which parameters create the most variation among these impacts. Environmental impacts here refer to emissions of carbon dioxide, nitrogen oxides, sulfur dioxide, mercury, and particulate matter (PM10); solid waste; nuclear waste; and water consumption. Using data from the US Department of Energy, the National Renewable Energy Laboratory, the US EPA Energy Star program, and the USGBC, among others, models using Monte Carlo analysis were created to simulate the range of impacts of LEED-certified buildings. Various metrics and statistics were calculated to highlight the significance of variation in these impacts. Future research needs and implications of the results for LEED version 3.0 are also discussed.