Chronic multisymptom illness is more prevalent among deployed than non-deployed Gulf War 1 veterans. Objective physiologic markers of chronic multisymptom illness are lacking. The purpose of this study is to determine whether measurable abnormalities in the autonomic nervous system or hypothalamic-pituitary adrenal axis would distinguish chronic multisymptom illness cases (CMI+) from controls (CMI-) among deployed veterans of the 1990-1991 Gulf War.