The symptoms of Gulf War syndrome are compatible with the hypothesis that the immune system of affected individuals is biased towards a Th2-cytokine pattern. Factors that could lead to a Th2 shift among Gulf War veterans include exposure to multiple Th2-inducing vaccinations under stressful circumstances and the way in which such vaccinations were administered, which would be expected to maximise Th2 immunogenicity. These factors may have led to a long-term systemic shift towards a Th2-cytokine balance and to mood changes related to the immunoendocrine state. Other vaccines that lead to similar long-term, non-specific shifts in cytokine balance are well-established. If our hypothesis is proven, treatment may be possible with regimens that induce a systemic Th1 bias.