The dopamine transporter gene (DAT1) has been extensively studied as one of the candidate genes in attention-deficit/hyperactivity disorder (ADHD). Several studies have reported on the association between the DAT1 10-repeat allele and cognitive variables in ADHD. However, few studies have been designed to ascertain the association between DAT1 genotypes other than the 10-repeat allele and cognitive endophenotypes in ADHD. The aim of this study was to examine the relationship between the DAT1 genotypes and the candidate endophenotypes, inattention and impulsivity symptoms, as measured by the continuous performance test (CPT), in a Korean sample of 85 children diagnosed with DSM-IV ADHD. Compared to the normal control group, the frequencies of the 9/10 genotype were significantly higher in the ADHD probands (chi(2)=13.45, p=0.02, OR=4.12, 95% CI: 2.21-12.34) and parents of probands (chi(2)=11.60, p=0.03). The 9-repeat allele frequencies were significantly higher in the ADHD probands (chi(2)=11.55, p=0.03, OR=4.43, 95% CI: 1.55-11.78) and parents of probands (chi(2)=12.70,p=0.03) than the normal control group. Compared to the ADHD probands without the 9-repeat allele (n=74), the mean T-score, with regard to the commission errors of the CPT, was significantly higher (p<0.05) in the ADHD probands with the 9-repeat allele (n=11). Compared to the ADHD probands with other DAT1 genotypes, the mean T-score, with respect to the commission errors of the CPT, was significantly higher in the ADHD probands with the 9/10 genotype (p<0.05). The results of this study suggest the possibility of an association between the DAT1 9-repeat allele and the impulsivity phenotype of ADHD.