Cross sectional echocardiography can identify anomalous origin of the left coronary artery from the pulmonary trunk. It has been suggested that identification of the left coronary artery arising from the aorta using this technique excludes the diagnosis. In three such infants the anomalous origin of the left coronary artery was identified in each by cross sectional echocardiography. In all three cases, however, an echo free linear structure apparently arising from the aorta, resembling a normal left coronary artery, was imaged. Anatomical sections in one patient, simulating cross sectional echocardiographic cuts, showed that this structure was almost certainly the transverse sinus of the pericardium. False positive cross sectional echocardiographic diagnosis of this condition is also possible because of the failure to image a normally arising left coronary artery. Thus identification of the anomalous origin of the left coronary artery from the pulmonary trunk appears to be the only reliable echocardiographic finding in this condition, and contrast cineaortography remains necessary in patients in whom the diagnosis is suspected clinically or electrocardiographically.