Background: Previous deployments like that to the Persian Gulf in 1991 produced veterans with post-deployment symptom-based health problems with no medical explanation. This was termed Gulf War illness or medically unexplained illness (MUI). If previous wars are any indication, some soldiers currently deployed to hostile areas also will return home with unexplained symptom-based illnesses. However, when this study began there was virtually no pre-war, prospective data on risk and resilience factors associated with MUI. This study is attempting to fill that gap.