BACKGROUND: One of the five level III neonatal intensive care units (NICU) in Finland has used prophylactic Lactobacillus GG (LGG) for very-low-birth-weight (VLBW) infants since 1997. AIM: To examine retrospectively the incidence of necrotizing enterocolitis (NEC) in all five university hospital NICUs in Finland in relation to the use of LGG during the years each unit has belonged to the Vermont Oxford Network (VON). METHODS: The incidence of NEC was analysed from the national database and from the VON databases separately in all five level III NICUs and additionally in three groups according to the probiotic practice in the hospitals: prophylactic LGG group, probiotics 'on demand' group and no probiotics group. RESULTS: The incidence of NEC was 4.6% vs. 3.3% vs. 1.8% in the prophylactic LGG group, the no probiotics group and the probiotics 'on demand' group [corrected] respectively; p = 0.0090, chi-square. LGG had no influence on the clinical course of NEC. CONCLUSIONS: The results of this retrospective report failed to show that LGG prophylaxis protects VLBW infants from the occurrence of NEC, in contrast to previously published results. Our results call for more research regarding effective ways to administer probiotics, including data on appropriate bacteria, strain, dose and timing of administration to achieve clinically robust effects.