“Complete” surgical correction of Fallot's tetralogy carries a much higher risk in patients with a hemoglobin concentration greater than 18 Gm. per cent (31 per cent mortality) than in those whose hemoglobin concentration is less than 18 Gm. per cent (10 per cent mortality).The successful reduction of hemoglobin concentration from greater than 18 Gm. per cent to less than 18 Gm. per cent through systemic-pulmonary anastomosis greatly reduces the surgical risk associated with “complete correction.” The mechanism by which this reduces surgical mortality is not known but may be related to (1) decreasing the hemorrhagic tendency associated with polycythemia and (2) conditioning of the pulmonary vascular bed and “left heart” in preparation for their new role when definitive repair has been accomplished.The hemoglobin concentration can be used to grade the severity of Fallot's tetralogy. Such grading would provide a basis for valid appraisal of surgical mortality rates among otherwise comparable series of cases.