A series of head-injured patients, in coma, were treated with hyperbaric oxygen (OHP) at 2.5 atm. Cerebral blood flow (CBF), cerebral metabolic rates of oxygen (CMRO2), glucose (CMRGL), and lactate (CMRL act), and various cerebrospinal fluid (CSF) parameters were measured before and 2 h after the treatment. Pre-OHP and post-OHP average values of arterial blood and CSF lactate, and CMRL act were higher than normal, while CBF, CMRO2 and CSF oxygen pressure (PO2) were lower. CBF tended to increase after OHP in some patients and to decrease in others. This discrepancy and the conflicting results of the literature can be tentatively explained in assuming that there is a different effect of OHP on normal brain circulation as compared to impaired brain circulation. Changes of cerebral metabolic rates were inconsistent and did not relate to changes of CBF, except with repeated studies of the same patient. A correlation was found between the variations of CMRGL and those of arterial blood and CSF glucose content. CSF PO2, CSF acid-base balance, and CSF lactate content did not vary, and arterial PO2 showed a consistent fall. In two patients who were neurologically improved after OHP exposure, the CBF and metabolic changes were not the same.