There is evidence that suggests that two omega-3 essential fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), are deficient in children with attention-deficit hyperactive disorder (ADHD).  The University of South Australia found that supplementation of EPA and DHA may reduce ADHD symptoms.  

The study involved 103 children with ages ranging from 7 to 12 that were diagnosed with ADHD.  The children were randomly given either a placebo, an omega-3 supplement, or an omega-3 supplement and a multi- vitamin.  After 15 weeks, all children were given fatty acid capsules and a daily multi-vitamin.   The children were evaluated after 15 and 30 week intervals using the Conners Parent rating scale, which is a test that measures ADHD symptoms. 
At week 15, the study noted that patients consuming the omega-3 supplement showed significant reductions in terms of hyperactivity, inattention, and impulsivity.  The study did not show any additional benefits to taking the multi-vitamin with the omega 3 supplement.  At week 30, the study recorded around half the children scored at least one standard deviation better on the Conners ADHD index.  To put this in perspective, the results are similar to the short-term effect of taking Ritalin; however, there is no evidence to suggest that changes found when taking Ritalin last beyond a 4 week period. 

The results of this study are far from conclusive, but this study is extremely encouraging.  Omega-3 supplementation has many other positive attributes and no long-term side effects.  When searching for an omega-3 supplement, it is important to find a high absorption rate.  With many omega-3 supplements, the pill does not fully dissolve and your body does not receive the benefits of omega-3.  We have found that Biometics' Biomega 3 is an excellent supplement for omega-3.  Here is a video from the creator of Biomega 3 explaining the benefits of taking this product..