The authors report their experience of echocardiography in cases of atrio-ventricular canal in the light of 9 cases which were studied by time-movement and 2-dimensional echography. In cases if ostium primum, the essential findings are: diastolic juxtaposition of the mitral valve to the left side of the septum, its multiple-echo appearance during systole, and the abnormal movement of the ventricular septum. In cases of complete atrio-ventricular canal, two main types are found. In the first, the echos of the atrio-ventricular valve are confused with those of the interventricular septum during diastole; in diastole, the septum disappears, one valve moves anteriorly into the right ventricle, and the other moves posteriorly into the left ventricle. In the second type, the appearances are those of a single valve which is situated within the left ventricle during systole, and which 'crosses' the interventricular septum to gain the right ventricle during diastole. The interpretation of these findings is difficult. It becomes clearer, however, when it is realised that the plane of incidence of the ultrasonic waves is not perpendicular to the plane of movement of the atrio-ventricular valve. The authors' conculsion is that echocardiography is a method of diagnosings atrio-ventricular canal defects, bud does not allow the anatomical type to be defined.