OBJECTIVE: To investigate the correlation between ABO blood type gene and attention-deficit hyperactivity disorder (ADHD) in children. METHODS: ABO blood types were determined using glass sheet method in 96 children with ADHD and their parents. O gene was identified using polymerase chain reaction and restriction fragment length polymorphism in patients with A or B blood type. Haplotype-based haplotype relative risk (HHRR), transmission-disequilibrium test (TDT) and Chi-square test were used to examine the association between ABO gene and ADHD. RESULTS: TDT results showed significant differences in the allele of ABO between the 96 children with ADHD and within-family controls(χ2=6.017, df=2, P<0.05). Chi-square test results showed differences in the allele of A and B (χ2=3.289, df=1, P=0.07) as well as O and B (χ2=3.629, df=1, P=0.057 ) between ADHD children and within-family controls. The frequencies of O and A genes were higher than that of B gene in ADHD children. CONCLUSIONS: There was correlation between ABO blood type gene and ADHD in children. The risk of ADHD is increased in the presence of alleles O and A, but the risk is reduced in the presence of allele B.［Chin J Contemp Pediatr, 2009, 11 (5):371-373］