More than 5,800 military personnel, mostly Navy personnel and Marines, participated in a series of tests of U.S. warship vulnerability to biological and chemical warfare agents, Project SHAD (Shipboard Hazard and Defense), in the period 1962-1973. Only some of the involved military personnel were aware of these tests at the time. Many of these tests used simulants, substances with the physical properties of a chemical or biological warfare agent, thought at the time to have been harmless. The existence of these tests did not come to light until many decades later. In September 2002, the Institute of Medicine (IOM) agreed to undertake a scientific study, funded by the Veterans' Affairs, of potential long-term health effects of participation in Project SHAD. In general, there was no difference in all-cause mortality between Project SHAD participants and nonparticipant controls, although participants statistically had a significantly higher risk of death due to heart disease, had higher levels of neurodegenerative medical conditions and higher rates of symptoms with no medical basis. Long-Term Health Effects of Participation in Project SHAD focuses on the potential health effects of participation in Project SHAD. It is a useful resource for government defense agencies, scientists and health professionals.