A 4.3 kg, three-month-old patient, diagnosed with a perimembranous ventricular septal defect, presented for cardiac surgery. Upon initiation of cardiopulmonary bypass (CPB), the patient developed carboxyhemoglobinemia (11.1%). Potential sources for the unexpected acquired carboxyhemoglobinemia were sought quickly. Testing of residual blood from the unit of packed red blood cells (PRBCs) used to prime the CPB circuit revealed a carboxyhemoglobin (COHb) of 15.1 %. A decrease in cerebral oximetry (rSO(2)) on CPB was initially felt to be a result of the elevated COHb levels. When ventilation of the oxygenator with 100% oxygen (O(2)) failed to decrease COHb levels, a partial exchange transfusion was performed with reduction in COHb to 7.1%. The operation was completed successfully and the patient's COHb levels returned to normal within 75 minutes. Post case analysis of events and data collected during the case revealed a broader differential for explaining the compromised patient's O(2) delivery than the transient acquired carboxyhemoglobinemia. A partial obstruction of the superior vena cava could have triggered the drop in rSO(2) on CPB. Follow-up of the donor blood confirmed the donor had previously undiagnosed carboxyhemoglobinemia as a result of chronic carbon monoxide exposure from a faulty vehicle exhaust system.