BackgroundChildren born extremely preterm disproportionately experience sequelae of preterm birth compared to those born at later gestational ages, including higher prevalence of autism spectrum disorder (ASD) and associated behaviors. AimExplore effects of combined dietary docosahexaenoic acid, eicosapentaenoic acid, gamma-linolenic acid, and oleic acid (omega 3-6-9) on caregiver-reported behavior and sleep in toddlers born at ≤29 weeks' gestation who were exhibiting symptoms commonly seen with ASD. Study design90-day randomized (1:1), double blinded, placebo-controlled trial. SubjectsThirty-one children aged 18–38 months received omega 3-6-9 (n = 15) or canola oil placebo (n = 16). Outcome MeasuresMixed effects regression analyses followed intent to treat and explored treatment effects on measures of caregiver-reported behavior (Child Behavior Checklist 1.5–5, Toddler Behavior Assessment Questionnaire – Short Form, Vineland Adaptive Behavior Scales, 2nd Edition) and sleep (Children's Sleep Habits Questionnaire, Brief Infant Sleep Questionnaire). ResultsTwenty-nine of 31 (94%; ntx = 13, nplacebo = 16) children randomized had data available for at least one outcome measure, 27 (87%; ntx = 12, nplacebo = 15) had complete outcome data. Children randomized to omega 3-6-9 experienced a medium magnitude benefit of supplementation on anxious and depressed behaviors (ΔDifference = −1.27, d = −0.58, p = 0.049) and internalizing behaviors (ΔDifference = −3.41, d = −0.68, p = 0.05); and a large magnitude benefit on interpersonal relationship adaptive behaviors (ΔDifference = 7.50, d = 0.83, p = 0.01), compared to placebo. No effects were observed on other aspects of behavior or sleep. ConclusionsFindings provide preliminary support for further exploration of omega 3-6-9 during toddlerhood to improve socioemotional outcomes among children born preterm, especially for those showing early symptoms commonly seen with ASD. Results need to be replicated in a larger sample. Trial registrationRegistered with ClinicalTrials.gov: NCT01683565.