To the purpose of better defining normative data on intracardiac dimensions and systolic time intervals in very low birth weight infants, we collected M-mode echocardiograms from 210 healthy preterm and term neonates with birth weights between 780 and 5,350 g and gestational ages ranging from 26 to 43 weeks. Fifty-nine neonates were less than 24 h, 62 were 25-48 h, and 89 were 48-144 h of age. Diastolic and systolic left ventricular dimensions increased gradually with advancing birth weight (r = +0.84 and 0.78). Left atrial and aortic root dimensions tended to show a parabolic relationship with birth weight, increments were reduced at higher birth weights (r = +0.92 and 0.85). The shortening fraction of the left ventricle (mean +/- SD 33.8 +/- 4.9%) and the left atrial/aortic ratio (1.16 +/- 0.10) were constant throughout all weight subgroups. Pre-ejection periods and ejection times of both ventricles were reduced in preterm infants due to their higher heart rates, but left and right ventricular PEP/ET ratios in preterm and term infants were comparable. Septal thickness in diastole and in systole tended to increase slowly with advancing birth weight, but correlation coefficients were low. This information is currently used as data base for a computer program to interpret M-mode echocardiograms performed in our nursery.