Cathechol-O-methyltransferase　(COMT)　regulates　the　amount　of　dopamine　in　the　prefrontal　cortex　(PFC).　Substantial　studies　indicate　a　close　relationship　between　COMT　and　several　human　psychotic　disorders.　The　case-control　method　was　used　to　study　the　association　between　mental　retardation　(MR)　and　genetic　variants　of　COMT.　Three　single　nucleotide　polymorphisms　(SNPs:　rs4680,　rs165656　and　rs165774),　in　the　cathechol-O-methyltransferase　(COMT)　gene,　were　genotyped　by　PCR-RFLP　method.　Individual　SNP　analysis　shows　significant　differences　only　at　SNP　rs165656　for　both　genotype　and　allele　frequency　when　comparing　MR　cases　and　controls　(p　=　0.023,　0.011,　respectively).　Further　haplotype　analysis　indicates　that　there　are　two　haplotype　sets,　rs165656-rs4680　and　rs165656-rs165774,　which　show　statistical　differences　between　MR　cases　and　controls　(global　p　=　0.047,　p　=　0.033,　respectively).　Our　results　suggest　a　positive　association　between　the　genetic　variants　of　the　COMT　gene　and　MR　in　the　Chinese　Han　population　in　the　Qinba　region.　(c)　2007　Published　by　Elsevier　Ireland　Ltd.