It has been suggested that certain characteristic symptoms of schizophrenia reflect a specific deficit in the ability to attribute mental states to others ('mentalizing'). Patients with negative features, particularly social withdrawal and blunted affect, those with thought disorder and patients with paranoid symptoms have difficulties when they try to infer what is going on in the minds of other people. This study examines this notion using two sets of cartoon jokes. While the first set can be understood purely using physical and semantic analysis, the second set requires that the viewer appreciates the mental state of the main character in order to 'get' the joke. For control subjects there was no difference in the ability to understand the two types of joke, while the schizophrenic patients found the mental state jokes significantly more difficult to understand. This effect was most marked in patients with behavioural disorders and those reporting passivity experiences. Those with paranoid delusions also showed a selective comprehension deficit with the mental state stimuli. Patients who were symptom free at the time of testing showed normal performance.