Breath-hold MR measurement of cardiac output was compared with results from respiratory triggered MR acquisitions, since flow measurement during breath-holding may be different from physiological blood flow. Cardiac output during large lung volume breath-holding (4.47 +/- 0.63 l/min in the aorta and 4.53 +/- 0.59 l/min in the pulmonary artery) was significantly lower than that measured during normal breathing (6.09 +/- 0.49 l/min and 6.48 +/- 0.67 l/min, P < 0.01). In contrast, no significant difference was found between measurements conducted with small lung volume breath-holding (5.87 +/- 0.53 l/min and 6.41 +/- 0.75 l/min) and normal breathing. In conclusion, breath-hold MR flow measurement using small lung volume by shallow inspiration can provide a blood flow quantification that is close to physiological blood flow. Magn Reson Med 45:346-348, 2001. (C) 2001 Wiley-Liss, Inc.