Quantitative flow measurements to determine forward flow in the ascending aorta were performed in 7 normal volunteers and in 6 patients by magnetic resonance imaging using an interleaved gradient echo pulse sequence. Cardiac output was determined simultaneously by a classical indicator dilution technique in the normals. In the patients, left- and right-ventricular stroke volumes were determined immediately before the flow measurement by a traditional spin echo multislice technique. There was a close correlation between aortic forward flow- and indicator dilution-derived cardiac output (r = 0.96, p < 0.001). Moreover, there was good agreement between aortic forward flow and left-ventricular stroke volume (r = 0.95, p < 0.01) and between aortic forward flow and right-ventricular stroke volume (r = 0.98, p < 0.001). The results indicate that the flow sequence is a reliable method for quantitative in vivo flow measurements.