['Down syndrome is frequently associated with complete atrioventricular septal defect. The aim of this retrospective study was to evaluate the impact of Down syndrome on morphologic features, surgical management, and perioperative and long-term mortality in patients with complete atrioventricular septal defect.', 'Between October 1974 and March 2005, 476 patients with complete atrioventricular septal defect underwent surgery, 341 (71.6%) of whom had Down syndrome. A total of 336 patients (70.6%) underwent primary repair, 95 patients (20%) underwent palliative surgery on an average of 3.4 +/- 2.9 years before repair, and 45 patients (9.4%) had no subsequent repair.', 'Patients with Down syndrome (group D) exhibited higher preoperative pulmonary artery pressure (P = .012) and resistance (P = .006) compared with patients without Down syndrome (group ND). Rastelli types B and C were more frequent in group D (P = .002), but left atrioventricular valve anomalies were more frequent in group ND (P = .03). No difference was seen regarding operative strategy (P = .117) and timing of repair (P = .32). Thirty-day mortality after complete atrioventricular septal defect repair was 4.9% in group D compared with 5.6% in group ND. Actuarial survival after 20 years is 84% +/- 2.5% (group D) and 75% +/- 5.7% (group ND) (P = .252), and freedom from left atrioventricular valve-related reoperation is 82% +/- 2.9% (group D) and 72% +/- 5.3% (group ND) (P = .004). Reinterventions on the left atrioventricular valve were required significantly more often in patients in group ND.'] Is the presence of Down syndrome a risk factor in complete atrioventricular septal defect repair?