The hemodynamic consequences of neonatal polycythemic hyperviscosity and the effects of partial exchange transfusion were evaluated in 13 infants. Mean (+/- SD) venous hematocrit was 72% +/- 2.5%. After partial exchange transfusion, whole blood viscosity at a shear rate of 11.5 sec-1 decreased from 16.2 to 8.4 centipoise. There were significant (P less than 0.05) increases in heart rate (127 +/- 7.5 to 139 +/- 7.8 beats/min), Doppler-derived cardiac index (200 +/- 35 to 263 +/- 48 ml/kg/min), left ventricular stroke volume index (1.56 +/- 0.23 to 1.89 +/- 0.33 ml/kg), systemic oxygen transport (51.4 +/- 8.4 to 57.9 +/- 11.9 ml/kg/min), and laser-Doppler peripheral (cutaneous) blood flow (+80%) after partial exchange transfusion. The increase in cardiac index probably resulted from reductions in pulmonary and systemic vascular resistance index, the latter decreasing from 0.26 to 0.19 mm Hg/ml/min/kg-1. The greater increase in cutaneous blood flow (+80%) versus cardiac index (+32%) after exchange transfusion suggests hemodynamic compromise and a redistribution of blood flow away from organs that use little oxygen during polycythemia. Our data provide a possible basis for the symptoms of neonatal polycythemia, and demonstrate the acute hemodynamic benefits of partial exchange transfusion.