To evaluate left ventricular (LV) mass in children with left-sided congenital diaphragmatic hernia (CDH), we retrospectively examined the echocardiographic data available on all newborn infants with a diagnosis of CDH between April 1989 and May 1993. Adequate data for evaluation were available for 20 of 31 patients with left-sided CDH and no significant congenital heart disease. Left ventricular mass was determined from two-dimensional echocardiograms by an area-length method. Findings were compared with a control group that consisted of neonates with other causes of pulmonary hypertension. Patients with left-sided CDH had a significantly lower indexed LV mass than control subjects (1.96 gm/kg +/- 0.59 vs 2.84 gm/kg +/- 0.41; p = 0.0001). Additionally, children with left-sided CDH who required extracorporeal membrane oxygenation before repair (n = 7) had a significantly lower indexed LV mass than those patients who did not require extracorporeal membrane oxygenation before repair (1.53 gm/kg +/- 0.50 vs 2.20 gm/kg +/- 0.52; (p = 0.007). Infants who survived (n = 13) had an indexed LV mass of 2.09 gm/kg +/- 0.58 vs 1.64 gm/kg +/- 0.58 in those who died (p = 0.07). We conclude that the LV mass index in children with left-sided CDH is significantly lower than in children with other causes of pulmonary hypertension in the newborn period. Evaluation of LV mass in neonates with left-sided CDH may help predict the need for extracorporeal support before surgical repair, and may help indicate overall prognosis.