Background: Leukocyte filters have been used in cardiopulmonary bypass circuits to decrease potentially deleterious consequences of white blood cell product release in adults undergoing cardiac operations. Studies in animal models have also demonstrated benefits of white blood cell depletion during cardiopulmonary bypass. We sought to examine the efficacy of a leukocyte filter placed in the bypass circuit of infants undergoing operation for correction of congenital heart disease. Methods: We conducted a retrospective study of infants undergoing correction of congenital heart defects (all had left-to-right shunts) between July 1993 and May 1994. Six infants (group I) had a Leukoguard-3 leukocyte filter (Pall Biomedical Products) placed in the arterial limb of the bypass circuit, and 6 infants (group II) had no filter. We analyzed for differences between groups in intraoperative variables (total pump time, aortic cross-clamp time, deep hypothermic cardiac arrest time, and time for rewarming), postoperative clinical variables (fever in the first 24 hours, days of mechanical ventilation, length of intensive care unit stay, and total hospital stay) and laboratory variables (change in white blood cell and platelet counts preoperatively to postoperatively, and arterial oxygen tension/inspired oxygen fraction ratio). Results: Comparisons between groups were made using Student's t test or χ2 where appropriate, and results were considered significant at p less than 0.05. No differences between groups were seen in the demographic data or in any of the intraoperative variables. Fever was the only parameter (clinical or laboratory) showing statistical significance between the groups, affecting 1/6 patients in group I and 5/6 patients in group II (χ2 = 5.33; p < 0.05). Conclusions: Although these data suggest that the leukocyte filter offers little clinical benefit, a prospective, randomized study should be undertaken to further elucidate the efficacy of leukocyte depletion for infants undergoing cardiopulmonary bypass for correction of congenital heart disease.