Gulf War syndrome (GWS) or Gulf War illness (GWI) is an illness reported by combat veterans of the 1991 Persian Gulf War typified by symptoms including immune system disorders and birth defects. It has not always been clear whether these symptoms were related to Gulf War service or the occurrence of illnesses in Gulf War veterans is higher than comparable populations.

Symptoms attributed to this syndrome have been wide-ranging, including chronic fatigue, loss of muscle control, headaches, dizziness and loss of balance, memory problems, muscle and joint pain, indigestion, skin problems, shortness of breath, and even insulin resistance. Brain cancer deaths, amyotrophic lateral sclerosis (commonly known as Lou Gehrig's disease) and fibromyalgia are now recognized by the Defense and Veterans Affairs departments as potentially connected to service during the Gulf War.

Since the end of the Gulf War, the United States Veteran Administration and the British Ministry of Defense have conducted numerous studies on Gulf War Veterans. The latest studies have determined that while the physical health of deployed veterans is similar to that of non-deployed veterans, there is an increase in 4 out of the 12 medical conditions reportedly associated with Gulf War syndrome (fibromyalgia, chronic fatigue syndrome, eczema, and dyspepsia.) They have also concluded that while mortality was significantly higher in deployed veterans, most of the increase was due to automobile accidents.