We assessed catechol-O-methyltransferase (COMT) polymorphism in 132 first-onset schizophrenic patients and 80 healthy controls. The relationship between COMT polymorphism and cognitive function, aggression and psychiatric symptoms was tested in the schizophrenic group. COMTL carrier had higher digit span score and lower similarity score than COMTH homozygote. COMTL carrier had higher attention and delusion scores and lower inappropriate affect scores than COMTH homozygote. Attention and delusion scores of COMTL allele were higher than COMTH allele. COMTL group had higher aggression than COMTH homozygote. Our results support the theory that COMTL allele was related with increased tonic dopamine activity and cognitive 'stability', which may induce cognitive inflexibility in schizophrenia.