Typically, blood loss after operations requiring cardiopulmonary bypass is estimated from the sum of blood on sponges and drapes, in the suction system reservoir, and in chest drainage bottles. Prime of the extracorporeal circuit is usually returned to the patient, but no accounting is made of blood remaining in the circuit. In 50 patients, we examined 25 bubble and 25 membrane oxygenator circuits after completion of cardiopulmonary bypass and after return of all prime to the patients. Saline solution was added to each circuit and recirculated for 3 minutes, after which the volume and hematocrit of the recirculated saline were determined. From these values, we estimated that 92.0 +/- 18.1 mL of red blood cells remained in the bubble oxygenators and 100 +/- 11.6 mL in the membrane oxygenators. This volume of red cells is equivalent to 250 mL of whole blood with 35% hematocrit or 30 grams of hemoglobin at 12 gm% concentration. We conclude that blood loss after operations requiring cardiopulmonary bypass is systematically underestimated by the approximately 250 mL left as red cells on the walls and filters of the extracorporeal circuit after return of all prime to the patient.