Attentional capacity and sustained attention were investigated in 21 aphasic stroke patients and 21 non-brain-damaged patients. Attentional capacity was assessed using a series of reaction time (RT) tasks. The aphasic patients demonstrated impaired attentional capacity as shown by slower processing speed than the non-brain-damaged group (p < .01) and greater increases in RT with increased processing load (p < .05). Similar patterns were found for both verbal and spatial material. There was no significant relationship between severity of auditory comprehension deficits and attentional capacity. Sustained attention was assessed using a cognitive vigilance task requiring identification of a target letter presented infrequently over 32 minutes. Both the aphasic and the non-brain-damaged group demonstrated a decline in performance with time on task as shown by a steady increase in RTs (p < .0001), but the decline was equivalent across the groups. Thus, the aphasic group did not show a specific deficit in the ability to sustain attention.