It has been reported that upwards of 50% of patients who survive an initial brain traumatic insult subsequently die due to infection and multiple organ failure. A paralysis of cell-mediated immunity following trauma, partially induced by anti-inflammatory cytokine release, appears to be responsible for the increased susceptibility to infections. We determined the plasma levels of the anti-inflammatory cytokine IL-10 and the pro-inflammatory TNF-alpha in 15 patients admitted with severe traumatic brain injury (TBI). None of the patients had received glucocorticoid or catecholamine treatment. Thirteen volunteers served as controls. At study entry the IL-10 plasma levels were significantly higher than in controls: 41.8 (17.3-265.4) pg/mL vs. 2.2 (1.4-2.7) pg/mL, p < 0.001 (Mann-Whitney test). There was no difference between the first (at study entry) and second sample (4 hours later) (Wilcoxon test). TNF-alpha plasma levels were similar in patients and controls at study entry and 4 hours later. We conclude that severe TBI patients present an early response, with a significant increase of IL-10 plasma levels. These results could partially explain the immunodepression following TBI.