Catechol-O-methyl transferase (COMT) plays an important role in the metabolism of neurotransmitters. Two alleles of the COMT gene as a result of a G/A transition in the exon 4 can lead to different COMT enzymatic activities. Much genetic research has revealed that this COMT functional polymorphism was related to human psychiatric disorders. Polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) methods were used to discern the relationships among the functional polymorphism of COMT, mental retardation (MR), and general cognitive ability of children. The results of the case-control analysis showed that there was no association between the frequencies of genotypes of COMT and MR (chi2=0.776, P>0.05) or between the frequency of COMT alleles and MR (chi2=0.335, P>0.05). COMT polymorphism was found in children whose intelligence quotient (IQ) was above 55. In normal children (IQ> or =85), the frequencies of high-activity allele COMTH and the homozygote genotype COMTHH were 60.98% and 79.28%, respectively. Both were higher than those of the borderline group (46.67% and 70.67%, 0.10 > P>0.05). Therefore, the result of this study suggests that this functional polymorphism is not an important risk factor for MR, but the COMTHH genotype may have a positive effect on cognitive performance in normal children in the Qinba area.