Hemodynamics after Norwood palliation for hypoplastic left heart syndrome (HLHS) have been incompletely characterized, although emphasis has been placed on the role that an excess pulmonary-to-systemic blood flow ratio (Q_p/Q_s) may play in causing hemodynamic instability. Studies suggest that maximal oxygen delivery occurs at a Q_p/Q_s <1. However, it remains unclear to what extent cardiac output can increase with increasing pulmonary perfusion. One approach is to use the oxygen excess factor Ω, an index of systemic oxygen delivery, and compare Ω with measured Q_p/Q_s. We measured Q_p/Q_s and Ω in neonates after Norwood palliation for HLHS, and determined how they were related. In addition, we determined the temporal course of surrogate indexes of systemic perfusion in the early postoperative period. Arteriovenous oxygen saturation difference, blood lactate, and Ω were recorded on admission and every 3 to 12 hours for 2 days in 18 consecutive infants with HLHS or variant after Norwood palliation. Three infants required extracorporeal membrane oxygenation (ECMO) 6 to 9 hours after admission. These infants had higher Q_p/Q_s, blood lactate, arteriovenous oxygen saturation difference, and lower Ω than non-ECMO patients. In non-ECMO patients between admission and 6 hours, Ω decreased significantly despite no appreciable change in Q_p/Q_s. We conclude that: (1) Oxygen delivery is significantly decreased at 6 postoperative hours unrelated to Q_p/Q_s. This modest decline in oxygen delivery is insufficient to compromise tissue oxygenation. (2) Patients requiring ECMO have significant derangements in oxygen delivery.