Background Straddling tricuspid valve, despite extensive investigation, remains an incompletely understood form of complex congenital heart disease. Methods A morphometric study of 19 postmortem cases of straddling tricuspid valve was performed, and the results were compared with 32 normal control heart specimens. Results In straddling tricuspid valve, marked malalignment of the ventricles was always found relative to the atria. The angle between the ventricular septum and the atrial septum in the short-axis projection averaged 61° ± 24°, the normal ventriculoatrial septal angle averaging 5° ± 2° (P < .001). The right ventricular sinus (inflow tract) was significantly smaller than the left (P < .01). A ventricular septal defect was present in 79%: atrioventricular canal type in 42%, atrioventricular canal type confluent with a conoventricular defect in 26%, and a conoventricular defect in 11%. When the straddling tricuspid valve adhered to the crest of the muscular ventricular septum (n = 4 cases, 21%), the 2 salient findings were (1) an intact ventricular septum and (2) double-outlet right atrium. The nonstraddling part of the tricuspid valve opened into the small right ventricle. The straddling part of the tricuspid valve opened into the larger left ventricle. The mitral valve also opened into the left ventricle. Hence hearts with double-outlet right atrium had 3 atrioventricular valves. Congenital mitral stenosis was present in 26% of this series. Conclusion Straddling tricuspid valve was always characterized by marked ventriculoatrial malalignment, indicated by an abnormally large ventriculoatrial septal angle, best seen in the short-axis projection. (Am Heart J 1999;138:1184-95.)