OBJECTIVES This study sought to characterize the echocardiographic features of straddling mitral valve (SMV) and to determine its surgical implications and midterm outcome in a large clinical cohort. BACKGROUND Despite a relatively large body of literature on the postmortem anatomy of SMV, there is a paucity of information regarding its echocardiographic features, surgical implications and preoperative predictors of outcome. METHODS A retrospective review identified 46 patients with SMV between 1982 and 1999 who underwent echocardiography and surgery and had follow-up data. A detailed review of the echocardiograms, surgical reports and all pertinent records was undertaken. RESULTS Review of the echocardiograms revealed a widely varying anatomy among the study patients. However, four distinct groups with relatively uniform morphologic features could be distinguished on the basis of segmental analysis. Cardiac malposition associated with right ventricular hypoplasia, superior-inferior ventricles and criss-cross atrioventricular relations were common among patients with {S,D,L} (S ϭ visceroatrial situs solitus, D ϭ D-ventricular loop, L ϭ L-malposition of the great arteries) (n ϭ 6) and {S,L,D} (n ϭ 5) segmental combinations but were rare among patients with {S,D,D} (n ϭ 26) and {S,L,L} (n ϭ 9) combinations. Surgical management consisted of a functional single-ventricle palliation in 38 patients (83%) and biventricular repair in 8 patients (17%). Overall mortality was 22%, but none of the seven patients who were operated on during the cohort's last five years (1994 to 1999) has died. By multivariate analysis, noncommitted ventricular septal defect was the strongest independent predictor of death (relative risk ϭ 10.2), followed by multiple ventricular septal defects (relative risk ϭ 4.7). CONCLUSIONS This study demonstrates that echocardiography provides detailed noninvasive imaging of the complex anatomic features of SMV and its associated anomalies. Anatomic classification based on segmental analysis allows the distinction of four groups with more uniform morphologic features. Although a biventricular approach is feasible in selected patients, a functional univentricular palliation is indicated in those with major straddling and markedly hypoplastic ventricles. (J Am Coll Cardiol 2001;38:819 –26) © 2001 by the American College of Cardiology Straddling mitral valve (SMV) is a rare congenital cardiac malformation that is invariably associated with a ventricular septal defect (VSD) and almost always with conotruncal anomalies (1–3). The etiology and morphogenesis of SMV remain incompletely understood, but its anatomic features have been well described on the basis of autopsy material (1– 8). These reports indicate that the mitral valve (MV) straddles the anterior aspect of the septum through a malalignment conoventricular-type VSD, that the most common associated conotruncal anomaly is double-outlet right ventricle (DORV) followed by transposition of the great arteries (TGA) and that left and right ventricular size varies considerably (2– 6). However, because of its rare occurrence, most published reports on SMV are based on a relatively small number of cases, and no consistent anatomic classification has been proposed. The morphologic and functional features of SMV in living patients have not been described in detail, and few reports have addressed the echocardiographic features of SMV (9 –12). Moreover, information on midterm outcome and preoperative echocar-diographic features predictive of mortality is scarce. Only a few surgical reports have included patients with SMV (13–16); the largest published clinical experience included only 11 patients (14). Thus, the spectrum of echocardio-graphic findings and the optimal surgical management of patients with SMV remain unclear. The goals of this study, therefore, are to propose a classification of SMV based on echocardiographic findings in 46 patients, to report their midterm clinical outcome and to identify preoperative variables predictive of mortality.