Data on hemodynamic findings in 19 infants and children with ventricular septal defect are presented. The ages of these patients ranged from 3 months to 14 years. Significant elevation of pulmonary arterial pressure was found in 17 of the 19 patients studied. Left-to-right shunts, at times of large magnitude, were found in all these patients. However, right-to-left shunts accounting for up to 50 per cent of systemic flow were also obtained, even in young patients. A history suggestive of cardiac failure was obtained in seven patients. Cyanosis was constant in one patient and was found intermittently in three others. A systolic murmur was heard in most cases in the third or fourth intercostal space. In four additional patients a diastolic murmur was also identified. The electrocardiogram provided valuable information, which correlated with the relationship of pulmonary and systemic arterial resistances, the status of intracardiac shunting of blood, and the total pulmonary blood flow. These factors are considered to be of paramount importance in selection of patients for the operative closure of ventricular septal defect. It has been demonstrated that the hemodynamic status of the patient may be difficult to assess at the time of cardiac catheterization because of the unpredictable effect of anesthesia on ventilation. The use of dye-dilution curves in the diagnosis and evaluation of both right-to-left and left-to-right shunts is a vital factor in the adequate assessment of the patients from the hemodynamic standpoint.