Adults with Prader-Willi syndrome (PAIS) and normal controls participated in a cognitive psychophysiology study in which event-related brain activity was recorded in a visual and auditory oddball task. In both tasks, participants were instructed to press a hand key to a target stimulus. All participants had successful task performance but persons with PWS had more target omissions than controls in the visual oddball task. The event-related brain activity in the PWS group revealed an abnormal deflation of the P3 component in both the visual and auditory oddball tasks. The findings support the notion that the auditory modality is more affected than the visual modality and of a short-term memory impairment in persons with PWS.