A group of 25 patients was studied by cardiac catheterization before and after open operation for isolated pulmonary stenosis by direct vision with use of cardiopulmonary bypass Organic infundibular obstruction, when present, was resected. The hypertrophied infundibulum, when it constitutes a severe obstruction, must also be resected. The average right ventricular systolic pressure preopratively was 124 mm. Hg and postoperatively it was 35 mm. Hg.The average gradient across the pulmonary valve preoperatively was 99 mm. Hg and postoperatively it was 17 mm. Hg, a mean decrease of 82 mm. Hg.The long-term hemodynamic response was distinctly superior in this group of patients operated upon under total bypass when compared with a similar series operated upon by us with a closed transventricular approach.The specific diagnosis of lesions causing obstruction to the outflow of the right ventricle is particularly crucial in selecting an operative approach if open operation by inflow stasis is utilized.Careful review of the selective right ventriculograms is an excellent preoperative guide in determining whether infundibular obstruction is present and hemodynamically significant.Patients having severe valvular stenosis, with right ventricular pressures above 140 to 150 mm. Hg should have an immediate one-third to one-half decrease in their right ventricular systolic pressures at the time of surgery, or infundibular resection should be done to reduce postoperative mortality.