Fish-oil supplements are marketed as enhancing intelligence and cognitive performance. However, empirical data concerning the utility of these products in healthy term infants is mixed, particularly with respect to lasting effects into childhood. We evaluated whether fish-oil supplementation during infancy leads to better neurocognitive/behavioural development at 6-years. We conducted a double-blind randomized, controlled trial of supplementation with omega-3 long-chain polyunsaturated fatty acid (n-3 LCPUFA) in 420 healthy term infants. Infants received either fish oil [containing at least 250 mg docosahexaenoic acid (DHA) and at least 60 mg eicosapentaenoic acid (EPA)] or placebo (olive oil) daily from birth to 6-months of age. Neurodevelopmental follow-up was conducted at a mean age (± SD) of 6-years ± 7 months, whereby 335 children were assessed for language, executive functioning, global IQ and behaviour. No significant differences were observed between the groups for the main neurocognitive outcomes. However in parent-report questionnaire, fish-oil supplementation was associated with negative externalizing (P = 0.035, d =.24) and oppositional/defiant behaviour (P = 0.006, d =.31), particularly in boys (P = 0.01, d =.45; P = 0.004, d =.40). Our results provide evidence that fish oil supplementation to predominantly breast-fed infants confers no significant cognitive or behavioural benefit to children at six years.