BACKGROUND: The main aim is to determine the incidence and associated factors of late onset sepsis (LOS) in very low birth weight infants (500-1500 g), from the NEOCOSUR Network during years 2001-2013. Secondary objectives are to describe the microbiology of the first and second episode of LOS and to study the association between catheter dwell time and LOS. METHODS: Demographic information and outcome data are prospectively and routinely collected across the network using predefined diagnostic criteria and online data entry. LOS was confirmed by isolation of the organism in blood or through cerebrospinal fluid in cultures. The participating countries were Argentina, Brazil, Chile, Paraguay, Peru and Uruguay. RESULTS: Overall incidence was 22.2% (3066/13,821). Infants who developed LOS were smaller by weight and gestational age; also, they feature less prenatal care and prenatal steroids, and longer hospital stays. A greater number of infants in the LOS group had 1 minute Apgar Scores ≤3. Multivariate logistic regression analysis showed a positive association between LOS and necrotizing enterocolitis, mechanical ventilation requirements, patent ductus arteriosus, oxygen dependency at 36 weeks and death. The majority of first LOS episode was caused by coagulase-negative staphylococci (44.3%). An increased risk of LOS was observed in relation to catheter dwell time (6% per day of stay of central lines). CONCLUSIONS: The incidence of LOS was associated with mechanical ventilation, patent ductus arteriosus, necrotizing enterocolitis and death. LOS was an important cause of morbidity and mortality in very low birth weight infants in our network, and coagulase-negative staphylococci was the most frequent causative microorganism.