The findings on echocardiography and their anatomical correlations are reported in a group of 15 neonates with the syndrome of hypoplasia of the left side of the heart. The lesions which make up this syndrome could be defined precisely using echocardiography. In the major forms of this syndrome (10 cases), the correlation between the clinical and the echocardiographical findings was sufficient to establish the diagnosis, and to avoid the necessity for cardiac catheterisation without prejudice to the treatment plan. Characteristic findings in the major types were: --an aortic diameter less than or equal to 5 mm; --a left ventricle which was absent or had a diameter of less than 11 mm with an LV/RV ratio of less than 0.6; --a mitral valve which was absent or had a very abnormal form with multiple echoes. The differential diagnosis on echocardiography and the limitations of the method are discussed.