Aim. The objective of this investigation was to describe changes in the psychopathological state and defence mechanisms of a group of patients suffering from psychotic disorders and receiving therapy in the Day Treatment Centre. Subjects and methods: The study group consisted of 55 people suffering from psychotic disorders. Most of them suffered from schizophrenia, some of them from schizophrenia spectrum disorder, bipolar disorder and organic psychotic disorder. In the course of therapy in the Day Treatment Centre, apart from receiving pharmacotherapy and individual care, the patients took part in an intensive psychosocial therapeutic programme. The study was a pre-post design. The first investigation took place two weeks after admission to the Day Treatment Centre and the second in the last week of therapy (the average time of hospitalization was 3 months). The psychopathological state was examined according to the PANSS scale. The defence mechanisms were investigated with the Defence Style Questionnaire 40. Results. The results of the study show statistically significant changes occurring both in the psychopathological state as well as defensive functioning of the patients during their therapy. Changes in the psychopathological state occurred in all the subscales of the PANSS scale: positive, negative and global, and in the overall evaluation. Statistically significant changes in defence mechanisms were present in the mature factor of defence mechanisms and in two separate immature mechanisms: autistic fantasies and displacement. A change in defense mechanisms did not influence changes that occurred in the psychopathological state. This could mean that Axis I in DSM IV and part of Axis II are at least partly independent. Conclusions. Defence functioning seems to be a useful construct in understanding changes that take place in the course of therapy. The Defence Style Questionnaire 40 is a practical instrument to measure conscious derivates of defense mechanisms.