This study was designed to define the prevalence and significance of infundibular obstruction following balloon pulmonary valvuloplasty. Thirteen of 62 children had infundibular gradients prior to valvuloplasty; five of these disappeared following balloon valvuloplasty. Five other children without pre-valvuloplasty infundibular gradients but with angiographic infundibular narrowing developed new infundibular gradients following valvuloplasty. Propranolol was administered to six children because of severe infundibular constriction, with improvement. None required surgical intervention. At follow-up the infundibular gradients either diminished or disappeared. The infundibular gradients appear to be more frequent with increasing age and severity of pulmonary valvar obstruction. Children developing systemic or suprasystemic right ventricular pressures after balloon pulmonary valvuloplasty may be candidates for propranolol therapy. Regression of the infundibular stenosis at follow-up can be expected, as has been observed after surgical pulmonary valvotomy. Because the infundibular obstruction can be successfully managed, balloon pulmonary valvuloplasty remains the treatment of choice for isolated valvar pulmonary stenosis. Use of balloon valvuloplasty in children less than 5 years of age and/or prior to development of pulmonary gradients in excess of 80 mm Hg may reduce the chance for development of infundibular reaction.