Haematocrit (HCT) and viscosity of whole blood were measured in ten polycythaemic hyperviscous newborn infants both before and after dilutional partial exchange transfusion with 5% albumin. This was performed in order to evaluate the effect on the lowering of HCT and whole blood viscosity. Mean umbilical HCT values decreased from 68.7% before, to 54.4% post transfusion. This decrease in HCT and viscosity was highly significant (P less than 0.001). Safety and lack of complications make human albumin solution superior to human plasma for exchange transfusion in neonatal polycythaemia.