Background: The Necrotizing Enterocolitis (NEC) is a devastating neonatal gastrointestinal emergency. The incidence is inversely proportional to the gestational age (prematurity) and birth weight. Although the diagnosis and treatment are fast, the best way to treat this pathology is the prevention. A recent meta-analysis shows that supplementing enteral feeding with a combination of probiotics can reduce the incidence of NEC and mortality related with NEC in VLBW (very low birth weight infants; <1500g) but it has not clear results, for lack of sample, in the ELBW (extremely low birth weight infants; <1000g)Objective: To evaluate the effectiveness of supplementing the feeding with probiotics (Lactobacillus Acidophilus and Bifidobacterium Bifidum) vs. placebo for the prevention of NEC in ELBW. The secondary objective is to asses the mortality, mortality related to NEC, and late-onset sepsisMethods: The design is a multicenter, randomized, double blind, placebo, parallel-group trial. 912 patients will be recluted during 3 years and randomizied into two groups. 456 will receive enteral feeding with probiotic supplementation and 456 will receive enteral feeding without supplementation