According to an article in the Scandinavian Journal of Nutrition (Scandinavia being where much of our purified fish oil supplements comes from), there is "mounting evidence" showing that fish oil DHA/EPA omega-3 fatty acids can be used in clinical treatment of ADHD.(1)
Several studies have reported that supplementation with fish oil at high dosages (16g per day - that's really high) was shown to provide significant results in behavioral patterns.(2)
1. Richardson, A.J. The importance of omega-3 fatty acids for behaviour, cognition and mood. Available at: http://www.ingentaconnect.com/content/tandf/ssnu/2003/00000047/00000002/art00008. Accessed on Oct. 18, 2008.
2. Sorgi, P.J., Hallowell, E.M., Hutchins, H.L, & Sears, B. "Effects of an open-label pilot study with high-dose EPA/DHA concentrates on plasma phospholipids and behavior in children with attention deficit hyperactivity disorder." Journal of Developmental & Behavioural Pediatrics; 2007, 28:82-91.