In previous work, we postulated that mean aortic flow velocity (MAFV) might be a direct measure of cardiac index. To investigate the assumptions inherent in this relation, we measured body surface area, aortic cross-sectional area (two-dimensional ultrasonography) and MAFV (Doppler ultrasonography) in 70 normal children. For a direct check of the relation, we simultaneously measured cardiac index (Fick technique) and MAFV (Doppler ultrasonography) in 25 children after cardiac surgery. In the normal group, we found that body surface area was directly proportional to aortic cross-sectional area (R 0.94), and MAFV at rest was the same in the ascending and descending aorta (t test, p < 0.05). In the intensive care patients, MAFV in the descending aorta was directly proportional to cardiac index over a wide clinical range [MAFV (cm/s) = CI (L/min/m2).7.7-1.2]. The assumptions made when deriving the relation between MAFV and cardiac index appear to be valid whether measured in the ascending or descending aorta. However, the scatter of results limits its clinical value. Mean aortic flow velocity is probably of greater use as a trend indicator and has the potential for continuous display using an esophageal Doppler probe when measured in the descending aorta.