OBJECTIVE: To evaluate the effect of oral Lactobacillus reuteri (L reuteri) first on the incidence and severity of Necrotising enterocolitis (NEC) and second on sepsis. DESIGN: Prospective randomised controlled study. SETTING: Tertiary neonatal intensive care unit. PATIENTS AND INTERVENTIONS: Preterm infants with a gestational age of ≤32 weeks and a birth weight of ≤1500 g were included (n=400). Infants in the first group were given 100 million CFU/day (5 drops) of lyophilised L reuteri (DSM 17938) mixed in breast milk or formula, starting from first feeding until discharge. Participants in the control group were given a placebo. MAIN OUTCOME MEASURES: To determine and compare the frequency of NEC and/or death after 7 days, frequency of proven sepsis, rates of feeding intolerance and duration of hospital stay. RESULTS: There was no statistically significant difference between groups in terms of frequency of NEC stage ≥2 (4% vs 5%; p=0.63) or overall NEC or mortality rates (10% vs 13.5%; p=0.27). Frequency of proven sepsis was significantly lower in the probiotic group compared to the control group (6.5% vs 12.5%; p=0.041). A significant difference was also observed with regard to rates of feeding intolerance (28% vs 39.5%; p=0.015) and duration of hospital stay (38 (10-131) vs 46 (10-180) days; p=0.022). CONCLUSIONS: Our results show that oral L reuteri does not seem to affect the overall rates of NEC and/or death in preterm infants followed up in the neonatal intensive care unit, and significant reductions were observed in the frequency of proven sepsis, rates of feeding intolerance and duration of hospital stay. TRIAL REGISTRATION NUMBER: NCT01531179.