In nine patients with left-sided congenital diaphragmatic hernia (CDH) developing within 24 hours of birth, the authors measured the dimensions of the bilateral main pulmonary arteries by echocardiography, and investigated whether the left:right main pulmonary artery dimension ratio (PAD ratio) was a useful index for predicting pulmonary hypoplasia and persistent fetal circulation (PFC). Echocardiography was performed shortly after admission, the PAD ratio was calculated, and the clinical course of each patient was determined. When the PAD ratio was approximately 1.0, patients did not suffer from PFC and had little evidence of pulmonary hypoplasia. When the PAD ratio was low, the patients suffered from PFC and had pulmonary hypoplasia on the side of the hernia. Thus, the PAD ratio measured by echocardiography appears to be a useful index for predicting pulmonary hypoplasia and the risk of PFC in patients with CDH.