Toxic or environmental exposures have been suggested as a possible cause of symptoms reported by Gulf War veterans. To further explore this hypothesis, we analyzed findings in 18,495 military personnel evaluated in the Department of Defense Comprehensive Clinical Evaluation Program. The program was established in 1994 to evaluate Persian Gulf veterans eligible for Department of Defense medical care who had health concerns after service in the Persian Gulf during Operation Desert Shield/Desert Storm. The evaluation included a structured clinical assessment, a physician-administered symptom checklist, and a patient questionnaire addressing self-reported exposures, combat experiences, and work loss. Among 18,495 patients examined, the most common symptoms were joint pain, fatigue, headache, memory or concentration difficulties, sleep disturbances, and rash. Symptom onset was often delayed, with two-thirds of symptoms not developing until after individuals returned from the Gulf War and 40% of symptoms having a latency period exceeding one year. There was no association between individual symptoms and patient demographics, specific self-reported exposures, or types of combat experience. Increased symptom counts were associated with work loss, the number of self-reported exposures, the number of types of combat experience, and certain ICD-9 diagnostic categories, particularly psychological disorders. Prolonged latency of symptom onset and the lack of association with any self-reported exposures makes illness related to toxic exposure less likely.