BACKGROUND: Left atrioventricular valve regurgitation (LAVVR) after atrioventricular canal (AVC) repair remains a significant cause of morbidity. Papillary muscle arrangement may be important. To investigate the implications of left mural leaflet morphology, we examined anatomic characteristics of the LAVV to determine possible associations with postoperative LAVVR. METHODS: All patients with biventricular AVC repair at our institution between January 1, 2011, and December 31, 2016, with necessary imaging were retrospectively reviewed. We assessed papillary muscle structure and novel measures of the left mural leaflet from preoperative echocardiograms and the degree of LAVVR from the first and last available follow-up echocardiograms. Associations with degree of early and late postoperative LAVVR were assessed with t tests, analysis of variance, or χ2 or Fisher exact tests, and multivariable logistic regression. RESULTS: There were 58 of 156 patients (37%) with significant (moderate or severe) early postoperative LAVVR, and 30 of 93 (32%) had significant LAVVR after 6 or more months. Fewer patients with closely spaced or asymmetric papillary muscles had moderate or severe late LAVVR vs those with widely spaced papillary muscles (17% vs 40%, P = .019). Controlling for weight at operation, genetic syndromes, and bypass time, widely spaced papillary muscles increased the odds ratio for late LAVVR to 3.6 (P = .026). Larger mural leaflet area was also associated with late LAVVR on univariable (P = .019) and multivariable (P = .023) analyses. One-third of patients with significant late LAVVR had no significant early postoperative regurgitation. CONCLUSIONS: Mural leaflet and papillary muscle anatomy are associated with late LAVVR after AVC repair. Late regurgitation can develop in the absence of early LAVVR, suggesting different mechanisms.