We conducted a 12-week, double-blind, placebo-controlled trial comparing the effects of high-dose eicosapentaenoic acid (EPA, 1.2ā€‰g) and placebo on cognitive function (continuous performance test) in nā€‰=ā€‰92 youth (age 6ā€“18-years-old) with Attention Deficit Hyperactivity Disorder (ADHD). Blood erythrocytes PUFAs were measured before and after treatment, to examine the effects of baseline endogenous EPA levels on treatment response and the effects of EPA treatment on PUFAs levels