OBJECTIVES: Impaired fetal growth is an independent cardiovascular risk factor and is associated with arterial wall thickening in children. No preventive strategy has been identified. We sought to determine whether dietary omega-3 fatty acid supplementation during early childhood prevents the association between impaired fetal growth and carotid arterial wall thickening. METHODS: The Childhood Asthma Prevention Study was a randomized, controlled single-blind trial in 616 children born at term, recruited antenatally from maternity hospitals in Sydney. Participants were randomized to either a 500-mg-daily fish oil supplement and canola-based margarines and cooking oil (omega-3 group), or a 500-mg-daily sunflower oil supplement and omega-6 fatty acid–rich margarines and cooking oil (control group), from the start of bottle-feeding or 6 months of age until 5 years of age. Carotid intima-media thickness (IMT), a noninvasive measure of subclinical atherosclerosis, was the primary endpoint of a cardiovascular substudy (CardioCAPS) at age 8 years. We examined the association of fetal growth with carotid IMT in children with birth weight <90th percentile (omega-3 group [n = 187], control group [n = 176]). RESULTS: In the control group, fetal growth was inversely associated with carotid IMT, but this was prevented in the omega-3 group (difference between groups of 0.041 mm [95% confidence interval 0.006, 0.075] per kg birth weight, adjusted for gestational age and gender, Pheterogeneity = .02). CONCLUSIONS: The inverse association of fetal growth with arterial wall thickness in childhood can be prevented by dietary omega-3 fatty acid supplementation over the first 5 years of life.