The equation that we proposed in this JOURNAL (1977), relating the postrepair ratio between peak right ventricular pressure and peak left ventricular pressure (PRV/LV) to the intraoperatively measured internal diameter of the narrowest point in the pulmonary arterial outflow tract (formerly called pulmonary "anulus"), performed well when prospectively tested in 25 patients undergoing repair of the tetralogy of Fallot. The observed PRV/LV in all 21 patients in whom no transannular patch was used fell within the 70 percent confidence limits (1 standard deviation) of that predicted from the measured diameter. A trivial difference (-0.03 +/- 0.069 PRV/LV units, P = 0.08) was found between predicted PRV/LV and that observed after repair, with body surface area (BSA) used for normalization. When the four additional patients who had secondary transannular patching are included, or where the normalization was with weight, the differences were not significant (p greater than 0.2). A slightly revised equation has been devised using the combined data from our original group and the patients used for prospective testing. This equation predicts with reasonable accuracy (r = -0.69, p less than 0.0001) postrepair PRV/LV without transannular patching from the intraoperatively measured diameter of the pulmonary arterial outflow tract. We believe it is therefore helpful in the operating room in making the important decision for or against using transannular patching.