To assess the effect of antihypertensive therapy on hemorheology in essential hypertension, blood viscosity and red blood cell deformability were examined in 45 patients with essential hypertension and 20 age-matched normotensive control subjects. Hypertensive patients were randomly assigned to monotherapy with five different antihypertensive drugs for 6 months and change of blood viscosity and red blood cell deformability were reexamined after the end of the monotherapy with each antihypertensive drug. Blood viscosity increased and red blood cell deformability decreased in hypertensive patients compared to normotensive control subjects. Monotherapy with each drug resulted in sufficient blood pressure control in all hypertensive patients. After the monotherapy with the alpha-blocker, terazosin, blood viscosity decreased significantly at shear rates from 22.5/sec to 450/sec, and red blood cell deformability, estimated by red blood cell filtration rate, increased by 15% (from 65 +/- 10 to 75 +/- 12 microL/sec, P < .05). The decrease in blood viscosity induced by alpha-blocker monotherapy may relate to an improvement of red blood cell deformability. It is possible that the treatment with alpha-blocker has a beneficial effect on the peripheral microcirculation due to an improvement of hemorheology in patients with essential hypertension.