The echocardiographic crux cordis is defined in normal subjects as the spatial interrelation between the atrial and ventricular septal planes and the plane of the atrioventricular (AV) junction. This crossroad can be visualized using apical, sub-costal, or precordial four-chamber echocardiographic projections. We studied this relationship in 33 patients with right AV valve atresia (29 with absent AV connection, 4 with an imperforate AV valve) and 14 with left AV valve atresia (9 with absent AV connection, 5 with an imperforate valve). An accurate diagnosis of AV valve atresia was successfully achieved in all patients, while the differentiation between an imperforate AV valve and absence of an AV connection was achieved in 45 out of 47 cases. Echocardiography demonstrated in some cases of absent AV connection, an arrangement which at first sight was analogous to that found in hearts with AV concordance and an imperforate AV valve. The differential diagnosis, therefore, must be based on the differing echocardiographic characteristics of the atretic atrial floor. In 31 out of 38 cases with absent AV connection it was possible to obtain simultaneous visualization of the ventricular septum and the atretic atrial floor. This proved helpful in planning the clinical management of these patients. The evaluation of crux cordis anatomy also differentiated AV valve atresia from other cardiac malformations with a similar presentation (valvular hypoplasia and common AV valve). It may prove the most reliable tool for the differential diagnosis of the spectrum of AV valve atresia.