Background: Temporary transition from the half-seated position (HSP) to the lying position (LyP) is often associated with an increase in intracranial pressure (ICP) during management of patients with severe traumatic brain injury (TBI). This study was designed to assess the impact of the temporary LyP on cerebral perfusion and oxygenation in cases of severe TBI. Method: Patients with a severe blunt TBI with indication of ICP monitoring were prospectively included. Patients underwent standardized management according to the international guidelines to minimize secondary insults. For each patient, a maneuver to a LyP for 30 min was performed daily during the first 7 days of hospitalization. ICP, cerebral perfusion pressure (CPP), mean velocity (Vm), pulsatility index (PI), regional cerebral oxygen saturation (rScO2), jugular venous oxygen saturation (SvjO2)) were compared in the HSP and the LyP. Results: Twenty-four 24 patients were included. The median Glasgow coma scale score was 6 (interquartile range (IQR), 3-8), the median injury severity score was 32 (IQR, 25-48), and the mean age was 39 ± 16 years. On day 1, ICP (+ 6 mmHg (IQR, 4-7 mmHg)) and CPP (+ 10 mmHg (IQR, 5-14 mmHg) were significantly increased in the LyP compared with the HSP. Vm increased significantly in the LyP on the mainly injured side (+ 6 cm/s (IQR, + 0-11 cm/s); P = 0.01) and on the less injured side (+ 4 cm/s (IQR, + 1-8 cm/s); P < 0.01). rScO2 behaved similarly (+ 2 points (IQR, + 2-4 points) and + 3 points (IQR, + 2-5 points), respectively; P < 0.001). Mixed models highlighted the significant association between the position and CPP, Vm, rScO2, with more favorable conditions in the lying position. Conclusions: Within the first week of management, the temporary LyP in cases of severe TBI was associated with a moderate increase in CPP, Vm, and rScO2despite a moderate increase in ICP.