Although infants with neonatal polycythemia and hyperviscosity often present with cardiorespiratory distress, little information is available regarding the cardiac function of such babies before or after partial exchange transfusion. To assess cardiac function, we performed M-mode echocardiograms in 19 asymptomatic newborn infants (4 to 12 hours of age) who had venous hematocrits greater than 65%. The echocardiograms were performed immediately prior to and following partial exchange transfusion and were repeated at 48 hours of age. Eighteen matched newborn controls also underwent echocardiography within the first 12 hours of life and again at 48 hours. Polycythemic newborns had elevated right ventricular preejection period to right ventricular ejection time ratios compared with controls (0.46 +/- 0.11 v 0.37 +/- 0.04, P = .002), suggestive of increased pulmonary vascular resistance. These indexes normalized following partial exchange transfusion. In addition, the polycythemic newborns were relatively bradycardic prior to exchange (116 +/- 13 beats per minute v 125 +/- 16 beats per minute, P less than .05), but heart rates normalized following the procedure. At 48 hours, the polycythemic and control groups were different only in that the mean shortening fraction of the polycythemic group was lower than that of the controls (32% +/- 4% v 36% +/- 6%, P = .02). The findings are consistent with elevated pulmonary vascular resistance associated with polycythemia and hyperviscosity. Cardiac output may be lower in polycythemic infants. The finding of reduced fractional shortening in polycythemic infants at 48 hours following partial exchange transfusion is unexplained.