Traumatic brain injury (TBI) induces local and systemic immunologic changes, release of cytokines, and cell activation. Perpetuation of these cascades may contribute to secondary damage to the brain. Therefore, the ability of the antiinflammatory mediator transforming growth factor-beta (TGF-β) to downregulate intrathecal immunoactivation may be of fundamental value for diminishing the incidence and extent of secondary insults. In this study, the release of TGF-β into cerebrospinal fluid (CSF) and serum of 22 patients with severe TBI was analyzed with respect to the function of the blood-brain barrier (BBB) for 21 days. Levels of TGF-β in CSF increased to their maximum on the first day (median, 1.26 ng/mL), thereafter decreasing gradually over time. Median TGF-β values in serum always remained within the reference interval (6.5 to 71.5 ng/mL). Daily assessment of the CSF-serum albumin quotient (Q_A) and of the CSF-serum TGF-β quotient (Q_TGF-β) showed a strong correlation between maximal Q_TGF-β and Q_A, indicating a passage of this cytokine from the periphery to the intrathecal compartment across the BBB. However, calculation of the TGF-β index (Q_TGF-β/Q_A) suggested a cerebral production of TGF-β in 9 of 22 patients. Levels of TGF-β could not be correlated with extent of initial injury by computed tomography (CT), CD4/CD8 ratios, acute lung injury, or clinical outcome as rated by the Glasgow Outcome Scale (GOS). Although increased levels of TGF-β in CSF seem to parallel BBB function, a partial intrathecal production is suggested, possibly modulated by elevation of interleukin-6 (IL-6). Thus, TGF-β may function as a factor in the complex cytokine network following TBI, acting as an antiinflammatory and neuroprotective mediator.