Global and regional cerebral blood flow (CBF) has been reported to be decreased immediately after head injury. Since neuropsychological deficits in patients with chronic head injury syndrome persist long after the initial trauma, we studied CBF, which reflects cerebral metabolism and activity, and evaluated the results of neuropsychological tests in 12 alert, responsive patients aged 18-26, both 2-13 months after head injury and 1-12 months after they regained consciousness. Global CBF was significantly decreased in patients with head injury relative to age-matched normal controls. Four patients had well-localized injury; in three of these, CBF over the affected region was significantly decreased relative to the same region in the opposite hemisphere and relative to the same region in an age-matched normal control. In one patient with right frontal injury, cognitive improvement on repeat testing was associated with a relative increase in right frontal CBF. The remaining eight patients had diffuse bilateral injury. Asymmetry ratios (larger value of mean hemispheric CBF divided by the smaller) were significantly higher for the group with localized dysfunction (1.2 +/- 0.08 vs. 1.0 +/- 0.01, p less than 0.01), suggesting homolateral decrease in CBF in this group. Three patients with diffuse injury underwent repeat studies 5-14 weeks later; all improved on psychological tests, and two had a significant increase in global CBF. These results suggest that the chronic sequelae of head injury include decreased CBF, presumably reflecting decreased cerebral metabolism, which correlates with the neuropsychological impairment.