Although values for cardiac output in the newborn infant have been reported previously, the methods utilized have been invasive. To assess if cardiac output could be determined noninvasively in the neonate, we measured mean ascending aortic blood flow velocity (VAo) in well newborns using a portable 5MHz, range gated, pulsed Doppler velocity meter. Measurements were made from a suprasternal approach in 8 preterm (mean birth weight 1718 gm; mean estimated gestation age 33.3 weeks) and 14 term (mean birthweight 3127 gm; mean EGA 39.8 weeks) healthy infants under one week of age. The internal ascending aortic systolic diameter was determined echographically and aortic cross sectional area was calculated: AAo = pi d2/4. Ascending aortic blood flow (QAo) was then computed as QAo (ml/min) = VAo (cm/sec) X AAo (cm2) X 60 (sec/min). With the exclusion of patent ductus arteriosus (PDA) and detectable intracardiac defects, QAo was taken to equal cardiac output. Flow determinations were normalized to body weight. The 8 preterm infants had a mean cardiac output of 221 +/- 56 (+/- SD) ml/min/kg. The 14 term infants had a similar mean cardiac output of 236 +/- 47 ml/min/kg. The mean cardiac output of all 22 infants was 230 +/- 50. This study establishes normal values for cardiac output determined noninvasively by the Doppler technique, in the first week of life in healthy infants. These values are similar to previously reported systemic blood flows, which were determined by cardiac catheterization and thermodilution methods in healthy newborn infants.