This paper describes an elementary deficit in sensory processing in people with schizophrenia. If paired sounds are presented to normal subjects, the response to the first sound, as measured by the P50 wave of the auditory-evoked potential, is much greater than the response to the second sound. The diminished response to the second sound is an example of a sensory gating mechanism that enables people to regulate their vigilance so that they can either detect all sounds in the environment or ignore most of them, in favor of narrowing the focus of their concentration. In schizophrenia, this mechanism is usually deficient; patients are in a state of hypervigilance and have diminished abilities to focus their attention. The deficiency appears to be genetically determined and to involve the brainstem control of sensory input to the hippocampus. Such sensory gating deficits may underlie more complex psychotic symptoms, such as hallucinations and delusions. Further studies of their neurobiology could lead to increased understanding of the pathophysiology of schizophrenia.