Our goal was to investigate the outcome of the left atrioventricular valve (LAVV) after repair of atrioventricular septal defects (AVSDs) by analysing the mechanisms of LAVV failure. A retrospective investigation of 202 children with AVSD, operated on between 1992 and 2016, was performed. Analysis focused on freedom from reoperation for LAVV dysfunction, with specific attention to the modes of failure. The population consisted of 129 (64%) patients with complete AVSD (cAVSD) and 73 (36%) patients with partial AVSD (pAVSD), corrected at a median age of 3.9 (interquartile range 3.4) months and 29.0 (interquartile range 90.4) months, respectively. Within a median follow-up period of 5.9 (interquartile range 12.6) years, 27 (13.4%) patients required reoperation for LAVV failure, respectively, in 17 (13.2%) patients with cAVSD and 10 (13.9%) patients with pAVSD. Freedom from reoperation for LAVV at 10 years was 87 ± 4% for cAVSD and 87 ± 5% for pAVSD (P = 0.789). The failure mode was predominantly technical in cAVSD (71% vs 20% in pAVSD), whereas residual anatomical anomalies entailed more frequently LAVV regurgitation (LAVVR) in pAVSD (80% vs 29% in cAVSD) (P = 0.018). Cleft suture dehiscence (n = 8) and incomplete cleft closure (n = 6) were considered technical failures, whereas remnant distortions of the subvalvular apparatus (n = 9), small asymmetric bridging leaflet (n = 2), double orifice (n = 1) and additional cleft (n = 1) were considered anatomical lesions. The incidence of repair at the 1st reoperation was 92.6%. Eight patients needed a 2nd and 2 patients a 3rd repeat LAVV operation. Five patients finally ended with a mechanical prosthesis. In the long term, patients with AVSD are still subject to LAVV failure. Despite systematic cleft closure, patients with cAVSD develop LAVV failure mainly because of a recurrent/residual deficit at the cleft, amenable to technical improvement. Patients with pAVSD need increased attention for additional anatomical features of the LAVV at the time of primary repair.