Neutrophils play a key role in the whole body inflammatory reaction occuring during cardiopulmonary bypass (CPB). Effects of leukocyte filtration with PALL LG-6 filter has been investigated clinically during CPB in congenital heart surgery in two groups of patients. Filter group consisted of 14 patients, age varied between 1.5-11 (mean 6.75) years and body surface area (BSA) 0.76 m². Control group consisted of 13 patients, age varied between 2.5-13 (mean 4.19) years and BSA 0.61 m². LG-6 filter was placed serially on the arterial line in filter group and blood samples were collected before the start of CPB, 5. minute of CPB, after the release of aortic clamp, after the end of CPB and 24 hours postoperatively. Hematologic, enzymes (Elastase, LDH, AST, ALT, CPK) and clinical parameters were evaluated and compared between two groups. There was a prominent increase in number of leukocytes, percentage of neutrophils and LDH, SGOT, SGPT, CPK enzyme levels in both of groups. Elastate enzyme level, which is one of the specific markers of neutrophil activation during CPB, decreased after the rise in the first phase of CPB in the filter group. On the other hand, a significant increase in elastate enzyme level during whole course of CPB was observed in the control group (p<0.05). This suggested that destructive effects of this enzyme was probably less at least at cellular level in most of the vital organs in filter group, where we found less elastate release as we compare it with the control group. We could not detect any clinical difference in the damaging effects of neutrophil activation during and after CPB between the two groups. But in complex cases with longer CPB time, especially in infant open heart surgery, benefits of neutrophil filtration can be obvious not only at cellular level but also in the clinical outcome of the patients with the effect of decreased neutrophil activation and elastate enzyme release. Therefore it is seems reasonable to use PALL LG-6 leukocyte filter during CPB in congenital heart surgery.