In the 1990s some veterans who participated in the SHAD tests expressed concerns to the Department of Veterans Affairs (VA) that they were experiencing health problems that might be the result of exposures in the testing. These concerns led to a 2002 request from VA to the Institute of Medicine (IOM) to carry out an epidemiological study of the health of SHAD veterans and a comparison population of veterans who had served on similar ships or in similar units during the same time period. In response to continuing concerns, Congress in 2010 requested an additional IOM study. This second study expands on the previous IOM work by making use of additional years of follow up and some analysis of diagnostic data from Medicare and the VA health care system.