Extravascular lung water (EVLW) was measured in 16 patients with congenital heart disease by the cold green dye, double indicator dilution technique. Five patients with optimally corrected tetralogy of Fallot served as controls, and EVLW in this group was 4.7 +/- 0.5 ml/kg (111 +/- 13 ml/m2) (mean +/- standard deviation). In 5 asymptomatic patients with atrial septal defect (ASD), normal pulmonary artery (PA) pressure and increased pulmonary blood flow, EVLW was 5.7 +/- 2.8 ml/kg (132 +/- 63 ml/m2), which was not significantly different from the value of control patients. However, in 6 patients with ventricular septal defect, PA hypertension, normal left atrial pressure and an equivalent left-to-right shunt to ASD patients, EVLW was 15.9 +/- 3.8 ml/kg (270 +/- 60 ml/m2). This was significantly different from values in both control and ASD patients (p less than 0.01). It is concluded that in the face of normal pulmonary vascular resistance, PA pressure is transmitted to the microvasculature, causing hydrostatic pulmonary edema. Other factors that may be implicated in the pathogenesis of pulmonary edema, such as increased pulmonary blood flow and relative lymphatic insufficiency in infants, cannot be excluded.