The interventricular septal defect in patients with tetralogy of Fallot is, in most cases, a nonrestrictive malalignment type of defect. Rare examples of a restrictive defect have been described. We report a case of tetralogy of Fallot with pulmonic atresia in which the interventricular septal defect was found to be nonrestrictive at birth. The obstruction of the defect developed gradually, until, at the age of 6 years, a gradient of 55 mm Hg across the defect was found at repeat cardiac catheterization. Echocardiography proved that the acquired obstruction was due to excessive tricuspid valve tissue, while Doppler studies suggested a similar gradient between the right ventricle and the ascending aorta. This rare case should alert the clinician to the possibility of an acquired obstruction of the ventricular septal defect, with suprasystemic right ventricular pressure, in cases of tetralogy of Fallot, even if an early study suggested a nonobstructive defect. The diagnosis of this situation is of importance because of the poor prognosis which it carries.