Background: Modified ultrafiltration has been shown to reverse haemodilution and improve ventricular function following cardiopulmonary bypass. There has been concern, however, about the safety and efficacy of modified ultrafiltration after the first stage of Norwood reconstruction for palliation of neonates with hypoplasia of the left heart and its variants. Methods: We reviewed the intraoperative course of all patients undergoing the first stage of Norwood reconstruction between September 1, 2000, and August 31, 2002. Results: The first stage of reconstruction was performed in 99 neonates, 78 with classical hypoplasia of the left heart, and 21 with variants. Mean weight at surgery was 3.1 plus or minus 0.7 kilograms. Genetic syndromes, weight less than or equal to 2.5 kilograms, and/or major additional cardiac or non-cardiac anomalies, were present in 44 patients. We deemed these patients to constitute the group at high risk. A modified Blalock–Taussig shunt was utilized in 95 patients, and a conduit from the right ventricle to the pulmonary arteries in 4. Deep hypothermic circulatory arrest was used in all patients for a mean period of 45 minutes, plus or minus 15 minutes. Total support time on cardiopulmonary bypass plus deep hypothermic circulatory arrest was 100 minutes plus or minus 26 minutes. Modified ultrafiltration was performed in all patients. The mean duration of modified ultrafiltration was 10 plus or minus 2 minutes, and the total volume of filtrate removed was 104 plus or minus 29 millilitres per kilogram. There were no complications from modified ultrafiltration, and no patient required discontinuation of modified ultrafiltration for haemodynamic instability. During modified ultrafiltration, the haematocrit increased from 31 percent plus or minus 4 to 46 percent plus or minus 6. Heart rate decreased from 170 plus or minus 17 beats per minute to 158 plus or minus 16 beats per minute. Systolic blood pressure increased from 57 plus or minus 12 to 63 plus or minus 13 millimetres of mercury, and diastolic blood pressure from 30 plus or minus 8 to 35 plus or minus 7 millimetres of mercury. All these values are significant at a p value of less than 0.0001. Hospital morality in the patients at low risk was 3 of 55 (5.5 percent), but was 12 of 44 (27.3 percent) in the patients deemed to be at high-risk. Conclusions: Modified ultrafiltration is safe procedure following the first stage of Norwood reconstruction, with improvement in all haemodynamic parameters measured. Modified ultrafiltration is an additional incremental strategy, which may contribute to the overall improvement in outcome following surgical palliation of patients with hypoplasia of the left heart or its variants.