This report presents a new, simple and reliable method of quantitating left-to-right shunts from arterial dilution surves. When indicatior (e,g., indocyanine green) is injected centrally (right venticle or pulmonary artery) and blood is withdrawn rapidly from either the aorta or other central systemic artery, in the presence of a left-to-right shunt to recorded indicator-dilution curve usually shows two discrete peaks: p-1 of the magnitude of the shunt expressed as percent of pulmonary blood flow, Experimental and clinical data demonstrate that this method is accurate in quantitating shunts from such curves.