BACKGROUND: Effects of probiotics on intestinal microbiota and feeding tolerance remain unclear in extremely low-birth-weight (ELBW) infants. METHODS: ELBW infants were randomly assigned to receive probiotics or no intervention. Stool samples were collected prior to, 2 and 4 weeks after initiation, and 2 weeks after probiotics cessation for infants in the probiotics group, and at matched postnatal age time points for infants in the control group. RESULTS: Of the 102 infants assessed for eligibility, sixty-two were included. Infants who received probiotics reached full enteral feeds sooner (Mean difference (MD) -1.8; 95% CI:-3.7 to -0.01 day), had a tendency toward lower incidence of hematochezia before hospital discharge (22.6% vs 3.2%; P = 0.053), and were less likely to require extensively hydrolyzed- or amino acids-based formulas to alleviate signs of cow's milk protein intolerance in the first 6 months of life (19.4% vs 51.6%; P = 0.008). Infants on probiotics were more likely to receive wide-spectrum antibiotics (64.5% vs 32.2%; P = 0.01). Multi-strain probiotics resulted in significant increase in fecal Bifidobacterium (P < 0.001) and Lactobacillus (P = 0.005), and marked reduction in fecal candida abundance (P = 0.04). CONCLUSION: Probiotics sustained intestinal Bifidobacterium and reduced time to achieve full enteral feeds in extremely preterm infants. Probiotics might improve tolerance for cow's milk protein supplements. CLINICAL TRIAL REGISTRATION: This trial has been registered at www. CLINICALTRIALS: gov (identifier NCT03422562). IMPACT: Probiotics may help extremely preterm infants achieve full enteral feeds sooner. Probiotics may improve tolerance for cow's milk protein supplements. Multi-strain probiotics can sustain intestinal Bifidobacterium and Lactobacillus until hospital discharge.