To explore the idea that deficits in source monitoring may underlie positive symptoms of schizophrenia, the current study compared schizophrenic patients' performance (n=15) on an internal source-monitoring task with that of normal controls (n=15). On the basis of a source-monitoring task in which participants had to recall whether they had verbalized answers or merely thought about these answers, overall source monitoring performance, discrimination index, and response bias were calculated. In addition, participants completed cognitive tests and symptomatology questionnaires. Relative to controls, patients had significantly more difficulties with monitoring their own actions and showed a tendency towards misclassifying imagined thoughts as verbalized thoughts. Source-monitoring performance was related to selective attention, but not to other cognitive domains. No relationship was found between source monitoring and symptomatology. Failures in internal source monitoring are a prominent feature of schizophrenia, and our results suggest that they form a more enduring characteristic of this disorder than has previously been assumed.