Based on the gaps in knowledge described above, we have conducted a 12-week double-blind, randomised, placebo-controlled trial (RCT) in youth (6–18 years) with ADHD, which: (1) compares a high dose of EPA (1.2 g/day) with placebo; (2) measures cognitive function as the primary outcome; (3) examines endogenous PUFAs levels to stratify patients and to examine the effects of treatments on PUFAs levels; and (4) investigate the inflammatory biomarker, high-sensitivity CRP (hs-CRP), and the neurotrophin, BDNF, as potential mechanisms. Here, BDNF is linked to attention deficit-hyperactivity disorder.