Whole blood viscosity at shear rates from 90 to 450 seconds(-1) was higher in 37 hypertensive patients than in a sex-matched group of 48 apparently normal adults from the same employed population (p < 0.05 to < 0.01). The 5% to 11% elevation in blood viscosity in hypertensive patients was similar to their elevation of peripheral resistance, suggesting a possible role of increased blood viscosity in the hemodynamics of hypertension.