The role of lipid peroxidation after brain injury is still not completely understood, and results of different studies have been equivocal. In this study, three proposed peroxidation markers were determined in patients early after isolated head injury and results compared to healthy controls. Malondialdehyde (MDA) and thiobarbituric acid-reactive substances (TBARS) were measured in plasma, and n-pentane was determined in patients' exhaled air. For MDA and TBARS no significant differences could be shown (0.267 vs. 0.358 ng/mL, and 0.896 vs. 0.814 ng/mL in patients vs. healthy volunteers, respectively). n-Pentane, however, was significantly increased in the expired air of patients (0.471 vs. 0.118 nmol/L in healthy volunteers). Similar results for n-pentane were obtained when only male patients and volunteers were considered (0.510 vs. 0.113 nmol/L). Stratification according to clinical outcome showed significantly higher values for n-pentane in male patients with poor outcome (0.656 nmol/L) in comparison with healthy male volunteers (0.113 nmol/L). No difference was found when patients were stratified according to the presence or absence of subarachnoid hemorrhage. It is concluded that, only in a sub-population of patients with brain injury, lipid-peroxidation is a crucial mechanism. n-Pentane seems to be a valuable marker to detect lipid peroxidation early after brain trauma. Malondialdehyde may be of value only later in the course of the disease. TBARS are not a specific marker and should therefore not be used.