Aim. The objectives of this investigation are to describe the changes in the psychopathological state and defense mechanisms occurring during therapy in a Day Treatment Center. Method. The study group consisted of 55 persons suffering from psychotic disorders. Most of them suffered from schizophrenia, some from schizophrenia spectrum disorders, bipolar disorder and organic psychotic disorder. In the course of therapy in the Day Treatment Center, apart from 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 into the Day Treatment Center and the second in the last week of therapy (average time of hospitalization was 3 months). The psychopathological state was examined according to the PANSS scale. Defense mechanisms were investigated with the Defense Style Questionnaire 40. Results. The results of the study show significant changes occurring, both in psychopathological state, as well as defensive functioning of the patients during the therapy. Changes in the psychopathological state occurred in all subscales of the PANSS scale: positive, negative and global. Changes in defense mechanisms occurred in the mature factor of the defense 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 psychopathological state - Axis I in DSM IV and defense mechanisms - part of the Axis 11 are partly independent elements. Conclusions. Defense functioning seems to be a useful construct in understanding changes occuring in the course of therapy. The Defense Style Questionnaire 40 is a practical instrument in measuring conscious derivates of defense mechanisms.