BACKGROUND Necrotizing enterocolitis (NEC) is the most common emergency of the gastrointestinal tract occurring in the neonatal period. There have been published reports which suggest that oral immunoglobulins IgA and IgG produce an immunoprotective effect in the gastrointestinal mucosa. This systematic review was undertaken to clarify the issue. OBJECTIVE To assess whether oral immunoglobulin administered to preterm and low birth-weight neonates reduces the incidence of necrotizing enterocolitis without adverse effects. METHODS The databases MEDLINE, CINAHL, Embase and the Cochrane Controlled Trials Register were searched. The text words 'necrotising enterocolitis OR necrotizing enterocolitis' AND 'immunoglobulin' with constraints 'neonate OR infant' were used. Proceedings of the Perinatal Society of Australia and New Zealand were hand searched. The computer neonatal discussion site 'Nicu Net' was also used. Additionally, all references in the identified trials were checked and authors were contacted to request any additional published or unpublished data. METHODS All randomized or quasi-randomized controlled trials where oral immunoglobulins were used as prophylaxis against necrotizing enterocolitis in preterm (<37 weeks gestation) and/or low birth-weight (<2500 gms) neonates. METHODS The procedures of the Cochrane Neonatal Review Group (CNRG) were used. The two reviewers independently assessed the trials for their methodological quality and subsequent inclusion in the review. Relative risk (RR), risk difference (RD), and number needed to treat (NNT) were used in the analysis. RESULTS Five studies on oral immunoglobulin for the prevention of necrotizing enterocolitis were identified of which three met the inclusion criteria. In this review of the three eligible trials (including a total of 2095 neonates) the oral administration of IgG or an IgG/IgA combination did not result in a significant reduction in the incidence of definite NEC [RR 0.84 (95% CI 0.57, 1.25), RD -0.01 (95% CI -0.03, 0.01)], suspected NEC [RR 0.69 (95% CI 0.42, 1.13), RD -0.01 (95% CI -0.03, 0.00)], need for surgery [RR 0.21 (95% CI 0.02, 1.75), RD -0.03 (95% CI -0.06, 0.00)] or death from NEC [RR 1.10 (95% CI 0.47, 2.59), RD 0.00 (95% CI -0.01, 0.01)]. CONCLUSIONS Based on the available trials, the evidence does not support the administration of oral immunoglobulin for the prevention of NEC. There are no randomised controlled trials of oral IgA alone for the prevention of NEC.