Covert orientation of visual attention was studied in four clinical groups: mild closed head injury (CHI mild), moderate to severe closed head injury (CHI mod/severe), dementia of the Alzheimer type (DAT), and Parkinson's disease (PD). Subjects rapidly pressed a button in response to one of two lateral peripheral targets. The target was preceded by a central cue that was either neutral, valid or invalid with respect to target location. A NoGo cue indicated that no response should be made. Whereas the normal control subjects of all age groups showed a benefit in reaction time (RT) to targets in expected locations and a RT cost when targets occurred in the unexpected side of space, both CHI groups showed reduced or no benefit but a normal cost. The PD group showed a reduced cost but normal benefit, and the DAT group were faster in response to targets after valid than invalid cues, but slowest to neutrally cued targets. Analysis of the amplitude of the contingent negative variation (CNV) and the amplitude and latency of the P3 related to targets supported the implication that different mechanisms of visual attention are impaired in these groups with different brain disorders. In the CHI groups, the focusing of attention during the cue-target interval was impaired. In DAT, division of attention was compromised, and in PD there was poor maintenance of attention. The impairments may be attributed to dysfunction of the distributed brain regions of the cortico-cortical network subserving visual spatial attention.