SUMMARY. Although M-mode echocardiography has become a valuable tool in the noninvasive diagnosis of hypoplastic left ventricle (HLV), it may not resolve all diagnostic uncertainty. This Study compares the findings of M-mode echocardiography, cross-sectional echocardiography, and autopsy in a group of 20 infants with HLV. M-mode echocardiograms alone were obtained in eight infants; five of these children underwent cardiac catheterization, and six underwent autopsy. Cross-sectional echocardiograms were obtained in the remaining 12 infants; two of these Underwent cardiac catheterization, and all 12 underwent autopsy. Cross-sectional examinations Were performed with either a high-resolution mechanical sector scanner or a sequentially scanning linear array. Short-axis views were used for left ventricular size and shape and for aortic root, pulmonary artery, and ductus size; suprasternal notch views were used for imaging the aortic arch and subxiphoid views for the atrial cavities. In all 12 infants the findings from crosssectional echocardiography closely matched those of autopsy with respect to degree of hypoplasia of the mitral valve, the left ventricular cavity, and the left ventricular outflow tract. If the diameter of a valve orifice exceeded 3 mm at autopsy, patency of the valve could be diagnosed from the Cross-sectional echocardiogram. The findings when M-mode echocardiograms alone were done less closely matched the autopsy findings, and in five of the eight cardiac catheterization was judged necessary to resolve remaining diagnostic uncertainty. This study found that crosssectional echocardiography provided an accurate noninvasive diagnosis of the spectrum of anatomy found in HLV and had fewer pitfalls than M-mode echocardiography.KEY WORDS: Echocardiography --Hypoplastic left ventricle m Noninvasive bIypoplastic left ventricle (HLV) comprises a variety of lesions and combinations of lesions that result in inadequate left ventricular output [6,8,13,16,21,26,31,32]. There is a spectrum of size for HLV, for the aorta, and for the degree of development of the aortic and mitral valves in HLV. At present HLV is inoperable, and M-mode echoCardiography has been useful in distinguishing it from potentially operable lesions that have similar clinical manifestations [5, 19]. Nevertheless, our experience with M-mode echocardiography in the blLV spectrum suggested that the variations produced considerable diagnostic uncertainty. The