Attention deficit hyperactivity disorder (ADHD) is a common disorder of the school-age population. ADHD is familial and genetic studies estimate heritability at 80-90%. The aim of the present study was to investigate the association between the genetic type and alleles for the monoamine oxidase (MAO) gene in Korean children with ADHD. The sample consisted of 180 ADHD children and 159 control children. We diagnosed ADHD according to DSM-IV. ADHD symptoms were evaluated with Conners' Parent Rating Scales and Dupaul Parent ADHD Rating Scales. Blood samples were taken from the 339 subjects, DNA was extracted from blood lymphocytes, and polymerase chain reaction was performed for MAO polymorphism. Allele and genotype frequencies were compared using the chi-square test. We compared the allele and genotype frequencies of MAO gene polymorphism in the ADHD and control groups. This study showed that there was a significant correlation among the frequencies of the rs5906883 (odds ratio [OR]=1.47, 95% confidence interval [CI]=1.08-2.00, p=0.014) and the rs3027407 (OR=1.41, 95% CI=1.03-1.91, p=0.029) alleles of MAO, but the final conclusions are not definite. Follow-up studies with larger patient or pure subgroups are expected. These results suggested that MAO might be related to ADHD symptoms.