The usefulness of subclassifying the anatomic variations of straddling and overriding atrioventricular (AV) valve by 2-dimensional echocardiographic observation were evaluated. Chordae straddling into a contralateral ventricle were subdivided into type A (chordae inserting into the contralateral ventricle near the crest of the ventricular septum), type B (chordae inserting along the contralateral ventricular septum) and type C (chordae inserting into the free wall or papillary muscles of the contralateral ventricle). Overriding AV valve anulus was described as minor (less than 50% of the anulus committed to the contralateral ventricle), major (about 50% of the anulus committed to each ventricle), and double-inlet ventricle (greater than 50% of both AV valves committed to a single ventricular chamber). In 52 patients straddling AV valve was diagnosed by echocardiography and confirmed by direct examination at surgery or autopsy. In 60 of 66 straddling AV valves (91%), the diagnosis and the degree of straddling (type A, B or C) were correctly identified by 2-D echocardiography. Major associated cardiac defects included double-outlet right ventricle (31%), complete transposition (23%), corrected transposition (19%), double-inlet ventricle (19%) and other complex defects (8%). Twenty-two patients (42%) had right-sided straddling, 21 (40%) had left-sided straddling and 9 (17%) had straddling of both AV valves. In 41% of the patients, straddling AV valve had a major impact on the type of surgery or the surgical outcome. Of these valves, 52% were type C (severe), 26% type B and 22% type A straddling.(ABSTRACT TRUNCATED AT 250 WORDS)