Our study tests the hypothesis that hyperviscosity independent of arterial O2content reduces cerebral cortical blood flow, O2delivery, and O2uptake. After baseline determinations, ten 2− to 4-day-old awake spontaneously breathing piglets were given an intravenous infusion (5 mlkg-1, body weight) of concentrated cyroprecipitate, whereas eight controls received normal saline. Cerebral cortical blood flow, arterial and superior sagittal sinus O2content, whole blood viscosity, hematocrit, blood gases, and plasma fibrinogen concentrations were measured at baseline and 3 h after infusion. No significant changes were observed in the control group. Three hours after the infusion of concentrated cryoprecipitate the experimental group showed an increase in whole blood viscosity, whereas hematocrit and arterial O2content were unchanged. There was a decrease in cerebral cortical blood flow and cerebral cortical O2delivery, whereas cerebral cortical O2uptake was unchanged. We conclude that hyperviscosity independent of arterial O2content reduces cerebral cortical blood flow and that although O2delivery was reduced in the newborn piglet cerebral cortical O2uptake was maintained. (Pediatr Res24: 486–489, 1988)