In November 1997, the Centers for Disease Control and Prevention (CDC) funded a study by the University of Medicine and Dentistry of New Jersey-Robert Wood Johnson Medical School to characterize and compare different approaches for defining the medically unexplained illnesses of Gulf War veterans. This study compared the symptoms of two groups of Gulf War veterans at two points in time. The study subjects included Gulf War veterans from the Department of Veterans Affairs Gulf War Registry who had participated in a previous study conducted by the New Jersey researchers, and veterans who had participated in the CDC Air Force study. Data-driven case definitions for illness previously derived from these two groups of Gulf War veterans were compared with standard or existing case definitions for unexplained multi-symptom illnesses (such as chronic fatigue syndrome, fibromyalgia, and multiple chemical sensitivity) to determine which definition best characterized Gulf War veterans' unexplained illnesses. The study also tried to determine the generalizability of the derived and existing case definitions in a new random sample of deployed and non-deployed Gulf War era veterans and active duty soldiers. The investigators are currently analyzing the data and preparing manuscripts for publication.