Objective: Catechol-O-Methyltransferase (COMT) enzyme plays an important role in dopamine metabolism. Three different genotypes have been defined, one resulting from a valine (val)-> methionine (met) amino acid change due to a guanine -> adenine nucleotide change at codon 108/158 of the COMT gene, while the other two genotypes are formed by alleles containing val (heat-resistant, high activity) and met (heat-sensitive, low activity). This functional polymorphism in the COMT gene causes an alteration in the enzyme activity, where the enzyme activity is reduced 4 times in genotypes related to low enzyme activity. In schizophrenic patients, alterations in the amounts and efficiency of dopamine are known to be related to the clinical symptoms and cognitive functions. This study aimed to investigate the effects of COMT Val158Met functional polymorphisms on clinical symptoms, executive functions related to prefrontal functions, and attention and verbal memory in schizophrenic patients.