Fatigue has been associated with illness in veterans of the Gulf War; however, few studies have confirmed self-reported fatigue by using clinical evaluation, and symptomatic veterans have not been evaluated with established criteria for Chronic Fatigue Syndrome (CFS). The authors describe the frequency and clinical characteristics of CFS in a sample of veterans residing in the northwestern United States. The sample was selected randomly from U.S. Department of Defense databases of troops deployed to southwest Asia during the Gulf War. The selected individuals were invited to participate in a clinical case-control study of unexplained illness. Of 799 survey respondents eligible for clinical evaluation, 178 had fatigue symptoms. Of the 130 veterans who were evaluated clinically, 103 had unexplained fatigue, and 44 veterans met the 1994 U.S. Centers for Disease Control criteria for CFS. In this population, the authors estimated a minimum prevalence of any unexplained fatigue to be 5.1%, and of CFS to be 2.2%. The estimated prevalence was greater among females than among males. Cases were similar to healthy controls, as determined by laboratory tests and physical findings. In comparison to several clinical studies of CFS patients, the authors of this study found a lower proportion of veterans who reported a sudden onset of symptoms (19%) vs. a gradual onset (50%). Although it has previously been suggested that veterans of the Gulf War suffer from higher rates of chronic fatigue than the general population, the study results described herein--on the basis of clinical examination of a population-based sample of veterans-actually indicate that an increased rate may indeed exist. Gulf War veterans with unexplained fatigue should be encouraged to seek treatment so that the impact of these symptoms on overall quality of life can be reduced.