Objective To determine whether very long–chain n-3 polyunsaturated fatty acids (PUFAs) affect illness and selected plasma cytokines in schoolchildren. Study design Thai schoolchildren aged 9 to 12 years consumed milk containing placebo (soybean) oil (n = 86) or fish oil (n = 94) on 5 days per week for 6 months; the latter provided 200 mg eicosapentaenoic acid plus 1 g docosahexaenoic acid daily. Episodes and duration of illness were recorded, and plasma interleukin (IL)-2 receptor, IL-6, IL-10, and transforming growth factor (TGF)-beta1 concentrations and the fatty acid profile of plasma phosphatidylcholine determined. Results After intervention, very long–chain n-3 PUFAs were higher in plasma phosphatidylcholine in the fish oil group than in the placebo group (P