In the present study, we investigated the association between the 4-repeat allele at the dopamine receptor D4 (DRD4) gene and the response to treatment with methylphenidate (MPH) in Korean children with attention deficit hyperactivity disorder (ADHD). The study subjects were 83 children with ADHD (8.40+/-1.73 years) who were recruited from two child psychiatric clinics in South Korea. All of the drug-naive ADHD children were treated with MPH for about 8 weeks. An improvement of more than or equal to [corrected] 50% in the ADHD Rating Scale-IV (ARS) scores after 8 weeks of treatment compared with the baseline ARS scores before the treatment was considered as a 'good response', whereas an improvement of less than [corrected] 50% was considered as a 'poor response'. After the genotyping for DRD4 was performed, we investigated the association between the genotype at DRD4 and the response to MPH treatment. We performed a comparison of the response to MPH treatment between the two largest groups, viz. the subjects with and without the 4/4 genotype at DRD4. According to the ARS scores of the subjects as assessed by their parents and by their teachers, we found that while 71.1 and 80.0% (32/45 and 24/30), respectively, of those with a good response to MPH treatment showed the 4/4 genotype at DRD4, only 31.6 and 37.7% (12/38 and 20/53), respectively, of those with a poor response to MPH treatment showed the 4/4 genotype at DRD4 (Pearson chi2-values=12.926 and 13.737, respectively, both df=1, and both p<0.01). Our findings support the existence of an association between the 4-repeat allele at DRD4 and good response to MPH in Korean ADHD children.