BACKGROUND Docosahexaenoic acid (DHA) has been associated with downregulation of inflammatory responses. OBJECTIVE To report the effect of DHA supplementation on long-term atopic and respiratory outcomes in preterm infants. METHODS This study is a multicenter, randomized controlled trial comparing the outcomes for preterm infants <33 weeks' gestation who consumed expressed breast milk from mothers taking either tuna oil (high-DHA diet) or soy oil (standard-DHA) capsules. Data collected included incidence of bronchopulmonary dysplasia (BPD) and parental reporting of atopic conditions over the first 18 months of life. RESULTS Six hundred fifty-seven infants were enrolled (322 to high-DHA diet, 335 to standard), and 93.5% completed the 18-month follow-up. There was a reduction in BPD in boys (relative risk [RR]: 0.67 [95% confidence interval (CI): 0.47-0.96]; P=.03) and in all infants with a birth weight of <1250 g (RR: 0.75 [95% CI: 0.57-0.98]; P=.04). There was no effect on duration of respiratory support, admission length, or home oxygen requirement. There was a reduction in reported hay fever in all infants in the high-DHA group at either 12 or 18 months (RR: 0.41 [95% CI: 0.18-0.91]; P=.03) and at either 12 or 18 months in boys (RR: 0.15 [0.03-0.64]; P=.01). There was no effect on asthma, eczema, or food allergy. CONCLUSIONS DHA supplementation for infants of <33 weeks' gestation reduced the incidence of BPD in boys and in all infants with a birth weight of <1250 g and reduced the incidence of reported hay fever in boys at either 12 or 18 months.