The exact incidence and risk factors for reoperation in partial and transitional atrioventricular septal defects are unclear. The goal of this study was to assess risk factors for left atrioventricular valve and left ventricular outflow tract reoperation in partial and transitional atrioventricular septal defects. All patients undergoing partial and transitional atrioventricular septal defects repair between 1995 and 2017 were reviewed. Patients were classified as infants (<1 year), toddlers (1-3 years), children (3-17 years), and adults (≥18 years). Survival and reoperation were assessed using log-rank test and Cox models for univariate and multivariable analyses, respectively. Overall, 265 patients underwent partial and transitional atrioventricular septal defects repair (partial: 177 67%). Median age was 2 years. The cohort included 73 infants (28%), 85 toddlers (32%), 94 children (35%), and 13 adults (5%). Trisomy 21 was present in 76 patients (29%), and in 216 patients (83%), the zone of apposition was completely closed. Perioperative mortality was 0.8%. Complete heart block did not develop in any patients. Ten-year survival and freedom from reoperation were 98% and 81%, respectively. On multivariable analysis, trisomy 21 (hazard ratio HR, 0.16) and older age compared with infants (toddlers: HR, 0.35; children: HR, 0.25) were protective for any reoperation, whereas heterotaxy (HR, 3.43) was a risk factor. For left atrioventricular valve reoperation, toddlers (HR, 0.35), children (HR, 0.25), and trisomy 21 (HR, 0.16) remained protective, whereas left atrioventricular valve anomaly was a risk factor (HR, 2.61). Likewise, for left ventricular outflow tract reoperation, toddlers (HR, 0.24) and children (HR, 0.06) were protective. Mortality after partial and transitional atrioventricular septal defects repair is minimal, yet reoperation for left atrioventricular valve disease and left ventricular outflow tract obstruction remains significant. Patients requiring repair during infancy are at higher risk of reoperation.