Clinical descriptions of perceptual and attentional anomalies in schizophrenia emphasize phenomena such as flooding, or inundation, by sensory stimuli. A failure of sensory "gating" mechanisms in the brain is hypothesized to account for these symptoms, and this hypothesis has led to a marked interest in their putative psychophysiological substrates. However, there are no systematic analyses of the phenomenology of these perceptual experiences, nor has the hypothesized connection between the clinical phenomena and their reported psychophysiological substrates been tested. In this investigation, a structured interview instrument was developed to measure perceptual anomalies as distinct from hallucinations and to determine their prevalence across sensory modalities in schizophrenia in 67 schizophrenia subjects and 98 normal controls. The instrument includes Likert ratings of hypersensitivity, inundation, and selective attention to external sensory stimuli. Good interrater agreement, determined from interviews, was obtained. Schizophrenia subjects had significantly higher auditory, visual, and combined scores (i.e., across all modalities) than normal controls did, indicating significantly more perceptual anomalies. For the schizophrenia group, the prevalence of auditory and visual anomalies was significantly greater than the other sensory modalities. The data indicate that the putative phenomenological correlates of sensory gating may be reliably measured and tested with the Structured Interview for Assessing Perceptual Anomalies.