Posttraumatic encephalopathy (PTE) is characterized by a combination of upper motor neuron, basal ganglia, cerebellar, and psychiatric disturbances. A "striatal" variant, with predominant posttraumatic parkinsonism (PTP), is uncommon and may be difficult to distinguish from idiopathic Parkinson's disease (PD). We report the clinical characteristics of six patients clinically presumed to have PTP who were investigated with 18F-dopa PET. We compared their findings with those of age-matched controls and a group of idiopathic PD patients without a history of head trauma. The PTP patients showed a uniform 40% reduction (p < 0.0001) of mean 18F-dopa uptake in caudate and putamen compared with controls. Their mean putamen uptake was significantly higher than that seen in the PD group (p < 0.004) while mean caudate uptake was lower. The PTP mean caudate:putamen Ki ratio (0.97) was similar to that of controls (1.10) but significantly lower than that of the PD group (1.90) (p < 0.0001). These results suggest that although PTP may appear clinically similar to PD, 18F-dopa PET may help to differentiate it in vivo by demonstrating uniform nigrostriatal involvement as opposed to relative sparing of caudate function. These data also provide support for the view that delayed neurologic sequelae may follow cumulative head trauma.