In this clinical trial on a sample of medication-naïve children and adolescents with ADHD, peripheral S100B levels were measured with two main objectives: (1) to establish comparisons with healthy children as controls; and (2) to observe gradual differences in serum S100B values in the ADHD group before (T0) and after (T3–T6) treatment with a triple therapy containing MPH, aMT and ω-3 PUFAs (EPA/DHA). The gene discussed is S100B; the disease is attention deficit-hyperactivity disorder.