Pulmonary compliance in infants was calculated by (1) determining tidal volume with a whole body plethysmograph and (2) substituting intraesophageal pressure for intrathoracic pressure. Compliance is expressed as the ratio of tidal volume to change in pressure measured at extremes of tidal volume. In normal infants there is a significant association between compliance and length. In infants with heart disease patent ductus arteriosus was associated with lowered compliance and after surgical correction there was marked improvement. Compliance was lower in ventricular septal defect (VSD), and following pulmonary artery banding values were normal with one exception. Diminution of compliance was related to peak pulmonary systolic pressure. Lesions associated with increased flow but normal pulmonary artery pressure or with pulmonary stenosis have a normal compliance. Serial determinations done on an outpatient basis give information of current and prognostic value.