Digitized M-mode echocardiography was used to evaluate left ventricular (LV) function in normal neonates and to document subtle abnormalities of LV function in 19 asymptomatic newborn infants with polycythemia and hyperviscosity. Echocardiograms were performed before and after partial exchange transfusion and at 48 hours of age. Results were compared to those obtained from analysis of echocardiograms from 18 normal infants with normal hematocrits at 12 and 48 hours of age. In the normal group there were no differences between the digitized indices of LV function at 12 hours and those at 48 hours of age. In the polycythemic group, prior to hemodilution, the peak rate of LV emptying was low compared to normal values (-3.1 +/- 0.7 sec-1 vs -3.6 +/- 1.0 sec-1, p less than 0.05) and normalized following the procedure to -3.7 +/- 1.3 sec-1. The peak rate of left ventricular posterior wall (LVPW) thickening also increased following partial exchange transfusion (3.2 +/- 0.6 sec-1 to 3.7 +/- 1.0 sec-1, p less than 0.05). At 48 hours of age peak rates of change for the left ventricle (-2.9 +/- 0.8) and LVPW (3.0 +/- 0.7) were abnormally low in the polycythemic infants, all of whom had undergone partial exchange transfusion. Also, at 48 hours, the LVPW time to peak rate of thickening and time to the end of thickening were prolonged. In addition, the peak rate of LV filling in diastole was low in the polycythemic group (3.0 +/- 0.9 sec-1 vs 3.9 +/- 1.3 sec-1, p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)