We studied six patients (5 paediatric, 1 neonate) treated with ECMO to quantify changes in inflammatory mediators (neutrophil elastase (NE), free radical activity (FR), interleukin 8 (IL8)) and total body water (TBW). Blood samples were taken before instigation of ECMO, 4, 12, 24 hours post-ECMO and daily for six days. FR activity was quantified using the oxidised IgG FI/UV ration. NE and IL8 levels were measured by ELISA. TBW was assessed by electrical bioimpedance. Statistical analysis was made using repeated measures analysis of variance and modified t-test where appropriate. Results are presented as mean +/- standard error of the mean. FR activity increased 4 hours after instigation of ECMO (IgG FI/UV 32.1 +/- 3.2 from 24.1 +/- 3.0 p = 0.005) and remained elevated. NE also increased by 4 hours (94.8 micrograms/L +/- 8.9 to 678 micrograms/L +/- 153.4, p = 0.005) but returned to pre-ECMO values by day 6. IL8 levels rose after ECMO (from 98 pg/ml +/- 39, to 24 pg/ml +/- 117.4) although no statistical difference was noted over time due to the large variation between subjects (p = 0.009). TBW (% pre-ECMO body weight) fell by 24 hours (from 118.6 +/- 12.6 to 96.5 +/- 8.2 p = 0.0004). This study demonstrated that ECMO stimulates an 'inflammatory' response to extracorporeal perfusion (increased FR, NE) but despite this, results in a reduction in total body water. The complex relationship between the inflammatory response to prolonged extracorporeal perfusion and its effect on tissue oedema merits further investigation.