The present study examined the integrity of the P300 component of the event-related potential (ERP) in patients at high imminent risk for schizophrenia in relation to healthy comparison subjects and patients in the recent-onset and chronic stages of schizophrenia. The P300 was recorded by using an auditory oddball task in 10 patients clinically considered at risk of being prodromally symptomatic for schizophrenia, 10 patients with recent-onset schizophrenia, 14 patients with chronic schizophrenia, 14 young healthy comparison subjects, who were age-matched to the high-risk and recent-onset schizophrenia groups, and 14 older healthy comparison subjects, who were age-matched to the chronic schizophrenia group. High-risk subjects displayed smaller than normal P300 amplitudes at the parietal, centroparietal and central scalp locations. The observed P300 amplitude abnormalities in high-risk subjects were severe, being comparable in magnitude to the abnormalities seen in recent-onset and chronic schizophrenia subjects. However, whereas high-risk subjects showed P300 amplitude abnormalities that were bilaterally symmetrical, subjects with recent-onset schizophrenia and, particularly, subjects with chronic schizophrenia exhibited abnormalities that were markedly larger over the left temporal scalp sites. Patients at high imminent risk for developing a first florid psychotic episode seem to manifest auditory P300 amplitude abnormalities that are similar, but not identical, to those observed in patients in the recent-onset and chronic stages of schizophrenia. These results support the idea that auditory P300 abnormalities in schizophrenia reflect a primary cognitive and pathophysiological feature of the illness.