OBJECTIVE: Cardiopulmonary bypass (CPB) causes significant morbidity in paediatric patients, yet the mechanisms involved in the related inflammatory processes (resulting in capillary leak and edema) are poorly understood. Moreover, earlier palliative and corrective intervention in neonates and infants has provided the cohorts of patients about whom little is known of their proinflammatory response. METHODS: In the present two group study, 14 neonates (age 1-28 days, 2.5-4.5 kg) and 13 infants (2-12 months, 3-7 kg), undergoing CPB for congenital heart disease were consecutively recruited. The two cohorts were well matched in terms of CPB and aortic cross-clamp times (P > 0.1). Blood samples were collected on induction of anaesthesia, 5 min following onset of CPB, at the end of CPB,and 30 min, 2 and 24 h post- protamine (PP) administration. Plasma concentration of cytokines interleukin-6 (IL-6) and interleukin-8 (IL-8), terminal complement complex (C5b-9) neutrophil counts and leucocyte elastase were measured. RESULTS: Plasma levels of all inflammatory markerss ignificantly increased in both groups during and following CPB as compared to baseline. During and following CPB the change in IL-8 level was more pronounced in neonates (peak 30 min PP, median (range): 1062 (182-3872) pg/ml) than in infants 568 (172-1368) pg/ml), P = 0.01. Changes in IL-6level were indistinguishable between groups intraoperatively, but remained significantly higher at 24 h in neonates (P = 0.02). Peri and postoperativelevels of C5b-9 were significantly higher in infants than in neonates (peak 30 min PP, median (range): 984 (118-1142) ng/ml vs 458 (22 1340) ng/ml inneonates respectively, P = 0.01) but were similar at 24 h. Despite this, leucocyte elastase profiles did not differ significantly between the respective cohorts. CONCLUSION: These results indicate that there may be differences between neonates and infants with regard to the inflammatory response to CPB and neonatal patients merit further investigation in order to elucidate whether the pathophysiology of their CPB related inflammatory response and its clinical sequelae differs from their older counterparts.