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This randomized, double-blind, placebo-controlled study was designed to determine the influence of carbohydrate supplementation on the granulocyte and monocyte response to 2.5 h of high-intensity running [76.7 [+ or -] 0.4% of maximal oxygen consumption ([VO.sub.2]max)]. Thirty experienced marathon runners ([VO.sub.2]max 53.4 [+ or -] 1.0 mL . [kg.sup.-1] . [min.sup.-1], age 41.5 [+ or -] 1.4 y) were randomly assigned to carbohydrate-supplement (n = 17) and placebo (n = 13) groups. Subjects rested for 10-15 min before a blood sample was taken at 0715, and then ingested 0.75 L carbohydrate beverage or placebo. At 0730 subjects began running at 75-80% of [VO.sub.2]max for 2.5 h, and drank 0.25 L carbohydrate or placebo fluid every 15 min. Immediately after the 2.5-h run (1000), another blood sample was taken, followed by 1.5-h, 3-h, and 6-h recovery samples. Carbohydrate supplementation had a significant effect compared with placebo on the pattern of change in plasma glucose and cortisol, and the blood concentration of neutrophils ([F.sub.[4, 112]]) = 5.13, P = 0.001) and monocytes ([F.sub.[4, 112]] = 4.78, P = 0.001), but not on blood granulocyte and monocyte phagocytosis or oxidative burst activity after 2.5 h of intensive running. Am J Clin Nutr 1997;66:153-9.