BACKGROUND: Preterm infants remain at high risk of adverse outcomes following necrotizing enterocolitis (NEC) and late onset sepsis (LOS). Meta-analysis of randomized trials has indicated a reduction in severe NEC following use of probiotics and bovine lactoferrin (LF). Overall, however, uncertainty remains over which probiotic, or combination to use. The aim of this study was to compare the incidence of severe NEC and LOS before and after routine supplementation with Lactobacillus GG (LGG) and LF. METHODS: In this retrospective cohort study, infants <32 weeks or <1500 g routinely received LGG and 100 mg lactoferrin daily from 2011 -2015 were compared with similar infants born from 2004-2008. Cases of NEC were Bell stage 2 or greater and LOS was blood or spinal fluid culture positive after 48 hrs of age. RESULTS: We noted a marked decline in the incidence of NEC from 3% to 1% with a RR of 0.29 (CI 0.1-0.9) and a number needed to benefit of 50. The cost of preventing one case of NEC was estimated to be NZ $2800, considerably lower than the cost of treatment. LOS rates were not significantly different. There was a decrease in retinopathy treatment rates. During the period there was one case of LGG sepsis in a 23 week gestation infant with abdominal pathology and one infant developed NEC after stopping prophylaxis. CONCLUSION: The rates of severe NEC was markedly reduced following prophylaxis. The case of LGG sepsis indicates caution is required in extremely preterm infants.