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Objective To compare mortality and morbidity in extremely preterm (EPT) and/or extremely low birthweight (ELBW) infants receiving or not receiving probiotic supplementation.Design Nationwide, population-based registry study.Setting All neonatal intensive care units (NICUs) in Norway.Participants All EPT (<28 weeks)/ELBW (<1000 g) infants born between 2014 and 2021, admitted to an NICU. Infants who within the first week of life died, underwent surgery for necrotising enterocolitis (NEC) or did not receive enteral feeds or started with probiotics after the first week of life were excluded from the main analyses. Logistic regression, propensity score matching, inverse probability of treatment weighting (IPTW) and clustering analyses were applied.Main outcome measures The primary outcome was all-cause mortality after the first week of life. Secondary outcomes were surgical NEC and late-onset sepsis after the first week of life, growth and probiotic sepsis.Results Among 1596 live-born EPT/ELBW infants, 1268 were included in the main analysis and 676 (53.5%) started probiotics in the first week of life. Probiotics were associated with lower all-cause mortality (IPTW analysis; adjusted OR (aOR) 0.65, 95% CI 0.43 to 0.97), but no reduction in surgical NEC (aOR 0.83, 95% CI 0.49 to 1.43). However, in an emulated intention-to-treat scenario, high (>80%) probiotic use versus low (< 30%) was associated with a reduction in surgical NEC (aOR 0.40, 95% CI 0.19 to 0.86). We found no association between probiotics and sepsis or growth. 5 out of 858 infants (0.58%) exposed to probiotics developed probiotic-associated sepsis, all survived.Conclusion Probiotic supplementation to EPT/ELBW infants was associated with lower all-cause mortality and possibly reduced NEC.