Magnetic resonance imaging (MRI) was used to investigate the size and the asymmetry of the planum temporale (PT) in 22 schizophrenic patients and 23 strictly matched healthy volunteers. The degree of thought disorder was related to the reduction of the physiological PT asymmetry. When thought disordered patients were contrasted with non-thought disordered patients and healthy controls for a measure of PT laterality, those with thought disorder showed a statistically significant loss of PT laterality. This finding suggests that thought disordered schizophrenics may be characterized by an abnormal development of cerebral lateralization in a region crucial for language processing.