Hemorheological factors play an important role in the pathogenesis of different cardiovascular diseases. The hemorheological and hemodynamic parameters in essential hypertension and their possible modification by antihypertensive treatment were examined in the following two studies. In the first study the fundus appearance and hemorheological parameters (plasma and whole blood viscosity (WBV), fibrinogen level) of 33 hypertensive patients (mean age: 55 years) were examined. The fundus appearance showed retinopathy in all the cases between stages I-III. All the measured hemorheological parameters of the examined patients were in the pathological range (WBV at 90 s(-1): 5.18 mPa s) and were significantly (p < 0.01) higher than in healthy controls (WBV at 90 s(-1): 4.18 mPa s). The hemorheological factors showed a parallel deterioration with the fundus appearance, namely their values were significantly (p < 0.01) higher in patients with a fundus appearance stage III (WBV at 90 s(-1): 6.02 mPa s) than stage I (WBV at 90 s(-1): 4.51 mPa s). These results show that there is a correlation between hemorheological parameters and fundus appearance in hypertensives, and this suggests that hemorheological factors may play a role in the development of hypertensive retinopathy. In the second study the hemorheological and hemodynamical effects of Doxazosin, a selective alpha-1-adrenoreceptor blocker agent, was examined in twenty patients (mean age: 54 years) with essential hypertension. Hemorheologic (hematocrit, fibrinogen, plasma and whole blood viscosity) and hemodynamic (cardiac output and index, total peripheral resistance) parameters and plasma lipids were determined. The measurements were carried out before the beginning of the treatment, after 1 week and after 12 weeks treatment periods. Besides significant reduction of blood pressure and total peripheral resistance (p < 0.001), a decrease in cholesterol (p < 0.001) and triglycerides (p < 0.01) levels and a beneficial effect on hemorheological parameters was detected. Fibrinogen and plasma viscosity decreased significantly (p < 0.01). Hematocrit value was also lower after one week (p < 0.001), then an increase could be seen. Whole blood viscosity showed similar changes as hematocrit, but the degree of its final increase was slighter, which was supported by the significantly lower value of corrected blood viscosity (p < 0.05). All these findings indicate that hemorheological factors may play a role in the pathogenesis and in the development of organ damages in hypertension.