The state of Massachusetts requested that the National Research Council evaluate a draft supplemental risk assessment prepared by the National Institutes of Health (see http://www.nems.nih.gov/aspects/nat_resources/programs/nepa2.cfm) associated with a proposed biocontainment laboratory at Boston University. This risk assessment is intended to form the scientific basis for an environmental impact report that was requested by the state. The National Research Council report indicates that the draft does not adequately develop worst case scenarios or alternative site comparisons for the release and spread of a pathogen. While the report commended NIH for working with the community to identify pathogens to include in the scenarios, this process appears to have led to the selection of pathogens that are not representative of a worst case scenario. A more acceptable analysis would have included agents that are readily transmissible and would have demonstrated that the modeling approach used recognizes biological complexities, reflects what is known about disease outbreaks, and is appropriately sensitive to population density.