Blood rheology was studied in 50 patients with a long history of essential hypertension, together with severe left heart hypertrophy (mass-volume relationship greater than 1.6) and angina pectoris, as well as in 17 patients with renoparenchymal hypertension. The rheologic findings were compared with those of 34 normotensive patients in whom coronary artery disease (CAD) was excluded by coronary angiography. Based on angiographic findings, the patients with essential hypertension could be differentiated into two groups: 20 hypertensive patients with normal coronary arteries and 30 hypertensive patients with coexistent CAD. In renoparenchymal hypertension, increased plasma viscosity (1.39 +/- 0.08 mPas) secondary to elevated fibrinogen levels (406.8 +/- 84.6 mg/100 ml) was found. Whole blood viscosity at low and high shear rates and the elastic component of blood were significantly more elevated in patients with renal hypertension than in patients with essential hypertension. In 30 patients with essential hypertension and coexistent CAD, higher levels of plasma viscosity (1.37 +/- 0.08 mPas, p less than 0.05) and fibrinogen (294.1 +/- 55.1 mg/100 ml, p less than 0.02) were found than in patients with essential hypertension and normal coronary arteries (1.32 +/- 0.07 mPas and 259.8 +/- 44.9 mg/100 ml, respectively). Hypertensive patients with normal coronary arteries, however, showed significantly higher levels of plasma viscosity, red blood cell aggregation, and whole blood viscosity than did normotensive controls. It is conceivable that increased blood viscosity in hypertensive patients with normal coronary arteries contributes to angina pectoris and to the reduction in coronary reserve that is observed in hypertensive patients (1).