We describe the role of increased eicosanoid production in inducing immunodepression associated with shock and trauma and the potential mechanisms by which prostaglandins produce immunosuppression. The effects of prefeeding with a diet high in n-3 polyunsaturated fatty acids (PUFAs) in preventing trauma-induced immunosuppression, and of such a diet on cellular immunity in patients after trauma are described. The potential mechanisms by which enteral diets containing n-3 PUFAs produce beneficial effects are also discussed. Although there is controversy concerning the immunomodulatory mechanisms of this diet, most of the information available indicates that dietary supplementation with n-3 PUFAs may prevent immunosuppression after trauma. Further studies are needed, however, to determine the precise composition and duration of supplementation with n-3 PUFAs that prevent the deleterious immune responses but do not alter normal immunoresponsiveness.