OBJECTIVE: Patients with schizophrenia experiencing delusions and hallucinations can misattribute their own actions to an external source. The authors test the hypothesis that patients with schizophrenia have defects in their ability to predict the sensory consequences of their actions. METHOD: The authors measured sensory attenuation of self-produced stimuli by patients with schizophrenia and by healthy subjects. RESULTS: Patients with schizophrenia demonstrated significantly less sensory attenuation than healthy subjects. CONCLUSIONS: Patients with a diagnosis of schizophrenia have a dysfunction in their predictive mechanisms.