Interest in the value of omega−3 (n−3) fatty acid supplementation for treatment of ADHD remains high. No prior meta-analysis has examined whether ADHD is associated with alterations in blood lipid levels and meta-analyses of supplementation have reached conflicting conclusions. We report two new meta-analyses. Study 1 examined blood levels of omega−3 fatty acids in relation to ADHD. Study 2 examined a larger sample of randomized intervention trials than previously reported. Study 1 included 9 studies (n=586) and found lower overall blood levels of n−3 in individuals with ADHD versus controls (g=0.42, 95% CI=0.26–0.59; p<.001). Study 2 included 16 studies (n=1408) and found that n−3 supplementation improved ADHD composite symptoms; using the best available rating and reporter (g=0.26, 95% CI=0.15–0.37; p<.001). Supplementation showed reliable effects on hyperactivity by parent and teacher report, but reliable effects for inattention only by parent report. Omega−3 levels are reduced in children with ADHD. Dietary supplementation appears to create modest improvements in symptoms. There is sufficient evidence to consider omega−3 fatty acids as a possible supplement to established therapies. However it remains unclear whether such intervention should be confined to children with below normal blood levels. •Meta-analysis found lower blood levels of n-3 fatty acids in ADHD versus controls.•Study verified efficacy with small effect of n-3 supplementation for improving ADHD symptoms.•Evidence may justify n-3 as a potential supplementary treatment for ADHD.