There have been conflicting reports on the association between the Val158/108Met polymorphism of the catechol-O-methyltransferase (COMT) gene and attention deficit hyperactivity disorder (ADHD). Therefore we would like to perform a meta-analysis of previous studies to assess the overall magnitude and significance of the association. Family-based and case-control studies of the association between the COMT gene polymorphism and ADHD were searched systematically and comprehensively. Odds ratios (OR) of association were pooled by the fixed effects model if no significant heterogeneity was present among different studies. Subgroup analysis by gender and ADHD subtypes were also performed. Eleven family-based and two case-control studies were identified. After pooling the results, no significant association between the COMT Vall58/108Met polymorphism and ADHD was found (OR 0.99 (95% CI: 0.88-1.12), P = 0.87). There was also no significant association when the results were stratified by gender or ADHD subtype. There was no significant statistical heterogeneity (chi2 = 12.27, P = 0.2) although clinical heterogeneity was present in the studies, especially the ethnicity of subjects. Sensitivity analysis demonstrated absence of undue influence of any single study. Standard regression analysis showed no significant publication bias. We concluded that no significant association was present between the most common COMT gene polymorphism and ADHD. Further studies should employ larger sample size in more homogeneous subjects. Further investigations in moderator variables and gene-gene and gene-environment interactions are also warranted.