Susceptibility to drug dependence and drug-induced psychoses is influenced not only by the pharmacological effects of the drug but also by the genetic factors of the individual. To clarify the latter, we investigated the association between methamphetamine (METH) dependence/psychosis and the hDAT1 gene (SLC6A3) encoding the dopamine transporter, which is the primary site of METH activity in the brain. Four exonic polymorphisms of the hDAT1 gene, 242C/T (exon 2), 1342A/G (exon 9), 2319G/A (3'UTR), and VNTR (3'UTR) were examined. Although there was no significant difference in genotypic and allelic distribution of the four polymorphisms between all METH dependence/psychosis patients (N=124) and controls (N=160), the patients with METH psychosis lasting for 1 month or more after discontinuance of METH consumption showed a significant excess of nine- or fewer repeat alleles of the VNTR in 3'UTR of the hDAT1 gene (P=0.0054, OR=4.24, 95% CI=2.46-7.31). The present study demonstrated that the presence of nine- or fewer repeat alleles of hDAT1 is a strong risk factor for a worse prognosis of METH psychosis.