Introduction : This study investigated the effects of a marine oil extract (PCSO-524®) on inattention, hyperactivity, mood and cognition in children and adolescents. PCSO-524® is a standardised lipid extract of the New Zealand green-lipped mussel and is an inflammatory modulator that inhibits the 5′-lipoxygenase and cyclooxygenase pathways and decreases concentrations of the pro-inflammatory arachidonic acid (AA). ; Methods : PCSO-524® or a matched placebo was administered for 14 weeks to 144 participants (123 males/21 females; mean age 8.7 years) with high hyperactivity and inattention in a randomised, double-blind, placebo-controlled study. The primary outcome was the Conners Parent Rating Scale assessing parental reports of behavioural problems. Secondary outcomes assessed changes in cognition and mood. ; Results : The results of the present study did not support the hypothesis that PCSO-524® improves parental reports of hyperactivity, inattention and impulsivity in children ages 6 to 14 years over placebo. Repeated measures ANOVA on post hoc subsample analysis indicated significant improvements in hyperactivity (p = 0.04), attention (p = 0.02), learning (p = 0.05) and probability of ADHD (p = 0.04) with a medium to large average effect size (d = 0.65) in those children who did not meet criteria for combined hyperactivity and inattention. Furthermore, significant improvements in the PCSO-524® group were indicated in a whole sample repeated measures ANCOVA on recognition memory between baseline and week 8 over placebo (p = 0.02, d = 0.56); this difference was not sustained at week 14. ; Conclusions : The results presented indicate that PCSO-524® may be beneficial in reducing levels of hyperactivity and inattention in a population of children with clinical and subclinical symptoms of ADHD.