Schizophrenia is a psychiatric mental disorder that seriously affects patient's life and the life of entire social communities. In Croatia it is assumed that the number of schizophrenic patients is around 20,000 and the results of epidemiological research point to the universality of schizophrenia, which means that it occurs in all cultures and socioeconomic communities regardless of development of the area where an individual lives, grows, operates and works. Impaired executive functions , which is characteristic in patients with schizophrenia are most likely a consequence of the impaired function of the dopaminergic system in the prefrontal cortex. Catehol-O-methyltransferase (COMT) is an important component within the prefrontal cortex, which is responsible for organizing and coordinating data coming from other parts of the brain. Variations in the COMT gene also may be associated with mental illness in people without 22q11.2 deletion syndrome. Researchers have looked extensively at the potential connection between changes in the COMT gene and the risk of developing schizophrenia. Most studies have focused on the effects of a particular commonvariation (polymorphism) in catechol-O-methyltransferase, so this research replicated it. Executive cognitive functions that rely on the integrity of the PFC, including working memory, cognitive flexibility, speed of procesing, attentions anf vigilance, verbal learning and memory, visual learning and memory, reasoning and problem solving and social cognition are imparied in schizophrenia. Impaired Wisconsin Card Sorting Test (WCST) performance has been one critical piece of evidence suggesting frontal lobe dysfunction in schizophrenia, so this research take the results on as a measure of the performance of cognitive operations under the management of executive functions in schizophrenic patients. Due to the lack of reliable data and the large number of conflicting results in literature, the aim of this thesis was to explore the association between the polymorphism Val/Val, Val/Met, Met/Met of the COMT gene and the cognitive capabilities (measured as a result at WCST) of 93 patients with schizophrenia, and if there is a difference in distribution of genomic COMT polymorphisms between male and female patients. For the first time, the association of three polymorphisms of the COMT gene (Val/Val, Val/Met and Met/Met) with deficit of executive function in patients with schizophrenia was explored with the logistical regression method. The results show that there are no statistically significant differences in the results of the Wisconsin card sorting test assay with respect to the COMT gene polymorphism of a schizophrenic person and there if no difference in distribution of genomic COMT polymorphisms between male and female patients. Understanding the role of genetic variants of the enzyme COMT, as an important link in the dopaminergic system, could greatly contribute to better understanding the dopaminergic system for the development of executive function deficits in schizophrenia. COMT gene polymorphism of a schizophrenic person.