Schizophrenic patients experiencing passivity phenomena believe their thoughts and actions to be those of external, or alien, entities. We wished to test the hypothesis that voluntary motor action in such patients would be associated with aberrant patterns of activation within the cerebral motor system. We used H2(15)O PET to study patients while they performed paced joystick movements on two occasions 4-6 weeks apart. During the first scan passivity symptoms were maximal, while by the second scan these symptoms had significantly improved in five of the seven patients. Two control groups were also scanned on two occasions: deluded schizophrenic patients without passivity phenomena and normal subjects. In normal subjects, performance of freely selected joystick movements with the right hand, compared with rest, revealed relative activation of prefrontal, premotor, motor and parietal cortical regions. Schizophrenic patients with passivity showed hyperactivation of parietal and cingulate cortices. This hyperactivation remitted in those subjects in whom passivity decreased over time. This reversible hyperactivity was not a feature of schizophrenics without passivity. Given that these hyperactive cerebral regions subserve attention to internal and external bodily space, and the attribution of significance to sensory information, they provide a plausible anatomical substrate for the misattribution of internally generated acts to external entities: the cardinal feature of delusions of passivity (alien control).