Background: Gulf War illness (GWI) affects up to 210,000 U.S. veterans who served in the Middle East during the Gulf War in 1990-1991. Symptoms include fatigue, muscle and joint pain, forgetfulness, headaches, rashes, and sleep disturbances. Routine exams cannot determine the cause of GWI. Researchers need more information to understand this disease.

Objective: This natural history study will look for differences in Gulf War veterans who experienced GWI and those who did not.

Eligibility: Gulf War veterans with GWI. Healthy Gulf War veterans who do not have GWI are also needed.

Design: Participants will stay in the NIH Clinical Center as an inpatient for 2 weeks. They will undergo many tests. Blood will be drawn many times throughout the study. Participants will also give urine, saliva, and stool samples. Scans to measure the brain, leg muscles, bone density and body mass will be done. They will have an exercise stress test and muscle strength tests. They will have a sleep study. They will have tests to look at how well the brain, heart and lungs are working. Participants will sleep in a specialized room that measures the amount of oxygen they use and the carbon dioxide they produce on four consecutive nights. A sample of fluid will be collected from inside the spine. Participants will take many surveys. Some will ask about their activities. Some will be about emotional and mental health. Some will be about thinking, memory, and behavior. Optional tests include other imaging scans and testing the autonomic nervous system. Samples of skin and muscle may be taken. After discharge, participants will wear activity monitors for 14 days. They will keep a diary of their symptoms, including fatigue, pain, and sleep, while wearing the monitors.