Study Abstract: In 1990 and 1991, the U.S. deployed approximately 700,000 troops to the Persian Gulf to liberate Kuwait from Iraqi occupation. While there were few casualties associated with the Gulf War, many individuals returned from this conflict with unexplained symptoms and illnesses. This constellation of symptoms has been termed Gulf War Veterans' Illnesses (GWI). Although several explanations have been offered as to the cause of GWI, none of the putative etiologic agents or conditions is currently supported by sufficient evidence. One explanation that has received fairly widespread attention is systemic Mycoplasma fermentans infection. It is the purpose of this study to determine if antibiotic treatment directed against Mycoplasma species (i.e. doxycycline) will improve functioning and symptoms in deployed Gulf War veterans with GWI.