Children (6-12 years) with attention-deficit hyperactivity disorder (ADHD) being treated with methylphenidate and standard behavior therapy for more than 6 months, whose parents reported no improvement in behavior and academic learning, were randomly assigned to receive supplementation with a combined ω3 and ω6 preparation or a placebo. Outcome was measured at 3 and 6 months after treatment using a self-assessment checklist completed by the parents. Statistically significant improvement was found in the treatment group compared with the placebo group (P < .01) in the following measures: restlessness, aggressiveness, completing work, and academic performance. Statistically significant improvement was not found at 3 months of treatment between groups but was evident at 6 months of treatment (P < .05) with inattention, impulsiveness, and cooperation with parents and teachers. Distractibility failed to show improvement. Effect sizes ranged from 0.3 to 1.1 at 3 months and 0.2 to 1.4 at 6 months for individual symptom variables.