Quantitative anatomic study of the hearts of eight infants with left-sided congenital diaphragmatic hernia (CDH) has revealed significantly decreased cardiac mass, due to hypoplasia of the left atrium and ventricle and interventricular septum. These morphologic deficiencies may be a result of compression of mediastinal structures by herniated abdominal viscera during prenatal life. Left ventricular hypoplasia is likely to be an important factor in the pathogenesis of cardiac insufficiency in patients with left CDH.