The application of the P300 component of the event-related brain potential to the study of attentional impairment in schizophrenia is discussed, and two recent studies are described. In one, the relative effects on P300 of stimulus modality and probability were evaluated. The data showed that the P300 is smaller in schizophrenic patients relative to normal controls for low-probability auditory stimuli. Next is described a preliminary report that evaluated whether this P300 reduction reflects a core deficit (trait marker) or clinical symptomatology (state marker). To pursue this question, a group of schizophrenic patients was studied on and off neuroleptic medication. The data showed that improvement in clinical state was highly correlated with increased visual P300 but was uncorrelated with auditory P300. These findings suggest that P300 elicited in the visual modality has the characteristics of a state marker of schizophrenia. In contrast, auditory P300 remains a candidate for a vulnerability trait marker of schizophrenia. The core deficit in schizophrenia thus appears to involve the auditory information-processing system, whereas fluctuations in clinical state may be reflected in the visual processing system.