Noninvasive monitoring of cardiac output can greatly facilitate the clinical assessment and management of neonates with cardiovascular compromise. To assess normal values of cardiac output in neonates, mean blood flow velocity was measured in the ascending aorta from a suprasternal approach using a range-gated, pulsed Doppler velocity meter, and aortic root diameter was determined from an M-mode echocardiogram. These techniques were combined, and cardiac output was evaluated in 59 healthy premature and 62 term newborn infants during the first week of life. Birth weights ranged from 780 g to 4,740 g and gestational age from 27 to 42 weeks. Cardiac output values increased linearly with advancing birth weight (r = +.94, P less than 0.001) and gestational age (r = +.95, P less than .001). Mean cardiac output values (+/- SD) per kilogram of body weight were 249 +/- 34 mL/min/kg and decreased with advancing birth weight: less than 1,500 g = 265 +/- 32 mL/min/kg; 1,500 to 2,500 g = 253 +/- 34 mL/min/kg; and greater than 2,500 g = 241 +/- 33 mL/min/kg. For clinical use, 325 mL/min/kg and 200 mL/min/kg can be used as upper and lower limits of normal, respectively. Doppler cardiac output estimates compared favorably with studies using invasive techniques.