Technical Abstract: To evaluate the health benefits of an exclusively human milk–based diet compared, with a diet of both human milk and bovine milk–based products in extremely premature infants. Infants fed their own mothers’ milk were randomized to 1 of 3 study groups. Groups HM100 and HM40 received pasteurized donor human milk–based human milk fortifier when the enteral intake was 100 and 40 mL/kg/d, respectively, and both groups received pasteurized donor human milk if no mother’s milk was available. Group BOV received bovine milk–based, human milk fortifier when the enteral intake was 100 mL/kg/d and preterm formula if no mother’s milk was available. Outcomes included duration of parenteral nutrition, morbidity, and growth. The 3 groups (total n = 207 infants) had similar baseline demographic variables, duration of parenteral nutrition, rates of late-onset sepsis, and growth. The groups receiving an exclusively human milk diet had significantly lower rates of necrotizing enterocolitis (NEC; P = .02), and NEC requiring surgical intervention (P = .007). For extremely premature infants, an exclusively human milk–based diet is associated with significantly lower rates of NEC and surgical NEC when compared with a mother’s milk–based diet that also includes bovine milk–based products.