Since the end of the Gulf War in 1991, many of the approximately 700,000 U.S. veterans have experienced undiagnosed illnesses. They attribute these illnesses to exposure to chemical warfare (CW) agents in plumes -- clouds released from the bombing of Iraqi sites. But in 2000, the Department of Defense (DoD) estimated that of the 700,000 veterans, 101,752 troops were potentially exposed. GAO was asked to evaluate the validity of DoD, the Department of Veterans Affairs (VA), and British Ministry of Defense (MOD) conclusions about troops' exposure. The GAO found that DoD's and MOD's conclusions about troops' exposure to CW agents, based on DoD and CIA plume modeling, cannot be adequately supported. The models were not fully developed for analyzing long-range dispersion of CW agents as an environmental hazard. The modeling assumptions as to source term data -- quantity and purity of the agent -- were inaccurate because they were uncertain, incomplete, and nonvalidated. The plume heights used in the modeling were underestimated and so were the hazard areas. Postwar field testing used to estimate the source term data did not realistically simulate the actual conditions of bombings or demolitions. Finally, the results of both DoD and non-DoD models showed wide divergences as to plume size and path. DoD's and VA's conclusion about no association between exposure to CW agents and rates of hospitalization and mortality, based on two epidemiological studies conducted and funded by DoD and VA, also cannot be adequately supported because of study weaknesses. In both studies, flawed criteria -- DoD's plume model and DoD's estimation of potentially exposed troops based on this model -- were used to determine exposure. This may have resulted in large-scale misclassification. GAO recommended that the DoD and the VA not use the plume-modeling data for any other epidemiological studies of the 1991 Gulf War. VA concurred with the recommendation; DoD and the CIA did not concur.