Chronic pain may impair performance on attentional processing capacity tasks. In the present study, event-related potentials were recorded to examine whether pain patients show performance decrements on attentional processing capacity tasks due to shared resources by pain and attention or, alternatively, due to deficits in allocating attentional resources during pain. Fourteen chronic pain patients and thirty age and education matched healthy controls were investigated. An attentional capacity probe task was used in which the difficulty level was manipulated, resulting in an easy and a difficult condition, while task-irrelevant visual probes were presented. These probe-elicited P3 amplitudes were assumed to provide the most pure estimate of processing capacity since they are relatively free from target-related processes. Event-related potentials were recorded from the midline electrodes Fz, Cz, Pz, and Oz. For the behavioral measures, it was found that pain patients maintained a different speed-accuracy tradeoff. Pain patients showed faster reaction time responses and higher error rates compared to controls. No significant differences were found between pain patients and controls on the primary task. Pain patients differed from controls with respect to amplitudes elicited by task-irrelevant probe stimuli. For healthy controls, the expected decreased amplitude was found for probe stimuli in the difficult compared to the easy task. In contrast, the pain patients did not show decreased probe amplitudes with increasing task load. The data may imply that allocation of attentional resources is deficient in pain patients, instead of attentional capacity.