Among Gulf War era veterans, data from the largest random survey presented by the U.S. Department of Veterans’ Affairs (VA) in 1998 (based on questionnaires completed by 11 216 deployed to the Gulf and 9 761 nondeployed) show that 5% reported chemical sensitivity among the nondeployed personnel and 15% reported the same among the deployed.(7) Other VA researchers report much higher rates—but the same 3-fold difference—in a smaller random sample of VA hospital outpatients: 86% of ill veterans deployed to the Gulf complained of chemical sensitivity, compared with 30% of undeployed ill veterans.(8) In the only study in which MCS was specifically assessed among veterans selected randomly from the VA Registry, investigators found 36% of 1 004 met common research criteria for MCS.(9) Among randomly selected Department of Defense (DOD) personnel who remain on active duty, two larger studies by the Centers for Disease Control found slightly lower—but still significant—2.1- and 2.5-fold increases in the prevalence of self-reported chemical sensitivity among those deployed to the Gulf, compared with those who were not deployed. In the “Iowa” study, in which the prevalence rates for deployed and nondeployed individuals were 5.4% and 2.6%, respectively, investigators used a detailed questionnaire to assess “probable MCS.”(10) In the “Pennsylvania” study,(11) in which prevalence rates were 5% versus 2%, respectively, only one “yes/no” question was asked about chemical sensitivity. Canadian Gulf War veterans reported only approximately one-half the prevalence of MCS (2.4%), but nevertheless this was 4 times more than their controls.(12) Even in the United Kingdom where MCS is little known, Gulf War veterans report being diagnosed with MCS at 2.5 times the rate of military controls.(13)