This is the first longitudinal cohort study of Persian Gulf War US soldiers to examine belief in exposure to chemical and biological weapons before and shortly after combat. A longitudinal sample of n = 1250 male Persian Gulf War US Army soldiers were surveyed 3 to 4 months before and 6 to 10 months after the 1991 War. Six to 10 months after combat, 4.6% of the cohort believed they had been exposed to chemical and biological weapons. Adjusting for demographics only, those who reported a greater number of combat exposures (odds ratio, OR: 18.8), or higher combat stress (OR: 12.27) were more likely to believe they were exposed. Adjusting for all variables soldiers who reported higher combat stress continued to be most likely (OR: 6.58) to believe they had been exposed to chemical and biological weapons. Individuals reporting higher combat stress are at substantially greater risk of reporting they have been exposed to chemical or biological weapons.