The formation of delusional beliefs is elucidated by the expressed experiences of 75 delusional patients. It is found that such beliefs may develop regardless of the presence (64%) or absence (20%) of anomalous experiences. Patients within the same diagnostic category remember it very differently. Many patients (71%) find a simple and reassuring way of understanding and relating to the external world by the formation of delusional beliefs. To many patients (47%) the delusional theme remains unchanged from the start and from one psychotic episode to another, while to other patients (24%) the theme changes in a hierarchical way. It is suggested that the formation of delusional beliefs is caused by impaired two-way communication or metathinking and that the delusional theme indicates the severity of cognitive dysfunction or mental disorder.