Objectives: In patients of tetralogy of Fallot (ToF) with absent infundibular septum, postoperative growth of the pulmonary valve annulus may be restricted because the patch for ventricular septal defect (VSD) closure is to be anchored to the pulmonary valve annulus. We hypothesized that deficiency of infundibular septum is a risk factor for the postoperative development of right ventricular outflow tract obstruction (RVOTO), especially when aggressive pulmonary annulus preservation strategy is employed. Methods: A retrospective review of 86 patients with ToF, who underwent surgical repair of ToF between June 1997 and August 2008, was performed. Median age at operation was 9.5 months. Infundibular septum was absent in 13 patients (subarterial-type VSD: 11, Total conal defect-type VSD: 2). Pulmonary valve annulus was preserved in 67 patients (67/86, 78%). Cox proportional hazards model was fitted to identify risk factors for trans-annular patch (TAP) upon initial repair and reoperation for postoperative RVOTO after pulmonary annulus preservation. Results: Median follow-up duration was 25.7 months (1 – 134 months). There was no early mortality and one late non-cardiac death. Preoperative pulmonary valve annulus size (Z-score) was the only risk factor for TAP (P=0.007), and deficiency of infundibular septum was not associated with increased risk of TAP (P=0.620). Among the patients with pulmonary annulus preservation, reoperation for RVOTO was performed in 4 patients (4/67, 6%) between postoperative 7 and 24 months, of whom infundibular septum was absent in 3. Subarterial or total conal defect type VSD was the only significant risk factor of reoperation for RVOTO after repair (P=0.006). Conclusions: Deficiency of infundibular septum is not associated with increased risk of TAP, but may be attributed to the increased risk of postoperative RVOTO after pulmonary annulus preservation. In ToF with deficient infundibular septum, pulmonary annulus preservation strategy should be employed with caution if the size of pulmonary valve annulus is marginal.