Objective: To examine the association between functional haplotypes in the catechol-o-methyltransferase (COMT) gene and ADHD diagnosis, and executive function (EF) in children with ADHD. Method: COMT single nucleotide polymorphism (SNPs; rs6269, rs4633, rs4818, and rs4680) were genotyped in 445 ADHD children. EF was assessed using Wisconsin Card Sorting Test (WCST), Tower of London, and self-ordered pointing task. COMT haplotypes were tested for association using family-based association testing (fBAT) and quantitative trait analyses. Results: fBAT analysis showed no association between COMT alleles/haplotypes and ADHD diagnosis and EF parameters. Using ANCOVA in the Caucasian only sample, significant associations between COMT haplotypes, and WCST indices were observed. However, after correction for multipletesting, the only significant effect observed was between rs6269 and the number of categories completed (a measure of concept formation ability) on the WCST, F(1,285) = 8.92, p = .003. Conclusion: These results tentatively implicate COMT gene in modulating EF in children with ADHD. (J. of Att. Dis. 2012; XX(X) 1-XX)