BACKGROUND: Pediatric cardiopulmonary bypass (CPB) often results in increased capillary permeability and accumulation of excess total body water, which can lead to multiple organ dysfunctions. Recently, modified ultrafiltration (MUF) has been used during pediatric cardiac surgery to limit various deleterious effects of CPB. We investigated the effects of MUF on lung compliance and gas exchange in pediatric open heart operations. METHODS: Fifteen pediatric open heart surgery patients were evaluated. Lung compliance (dynamic compliance), arterial oxygen tension (PaO2), arterial to end-tidal carbon dioxide tension difference (Pa-ETCO2), systolic blood pressure (SBP), heart rate, and hematocrit (Hct) were measured. Measurements were made after induction of anesthesia, after CPB, after MUF and after sternal closure. RESULTS: Lung compliance, PaO2, SBP and Hct increased significantly after MUF compared to after CPB. After sternal closure, lung compliance decreased significantly from the value following MUF. PaO2, SBP, Hct decreased significantly after CPB compared to the values obtained after induction of anesthesia. Pa-ETCO2 and heart rate did not change significantly. CONCLUSIONS: Modified ultrafiltration increases lung compliance, PaO2, SBP and Hct. MUF is an excellent option for improving cardiopulmonary function immediately following pediatric cardiopulmonary bypass.