Hematocrit (Hct) and whole blood viscosity was studied at a mean age of ten hours in 100 neonates. Group A (n = 25), were term normal newborns, Group B (n = 25) were preterms, Group C (n = 20) were term small for gestation (SGA) and Group D (n = 30) had perinatal hypoxia. Blood viscosity was estimated in all cases at shear rates 94.5, 51.2, 20.4 and 8.1 and intergroup variability in viscosity compared at shear rate 51.2. The mean hematocrit (Hct) (59.4%) and viscosity (8.2 cps) was higher in Group A as compared to other groups, but the difference was not significant (p greater than 0.05). The upper limit of viscosity in Group C (11.9 cps) was higher than in all other groups but this difference was also not significant (p greater than 0.05). With decrease in shear rates a reciprocal increase in viscosity was noted in all four groups. Seventeen neonates (17%) had polycythemia of which eight (47.5%) were SGA. Twelve per cent preterms were polycythemic. Only 3% of neonates had hyperviscosity. The mean Hct and viscosity of the 17 cases with polycythemia was 70.9 and 9.21 cps, respectively, which was significantly higher than mean Hct and viscosity of Group A (p less than 0.05). Partial exchange transfusions were done in five neonates with Hct greater than 75%, of which only one had hyperviscosity. Post-exchange viscosity was not estimated. Whereas, three neonates with polycythemia were symptomatic, none of these had hyperviscosity. A linear correlation between Hct and viscosity was observed (r = 0.67).