Increased blood viscosity reduces blood flow and elevates vascular resistance in the cardiovascular system. The aim of this study was to elucidate how blood viscosity could affect renal function and eventually contribute to renal damage in essential hypertensives (EHT). In 164 untreated EHT without apparent renal damage (96 men, 56±12 years old, creatinine clearance 123±33 ml min(-1) per 1.73 m(2) and urinary albumin excretion 19±19 mg per day), blood and plasma viscosity was determined using a falling ball microviscometer. Blood viscosity correlated negatively with creatinine clearance (r=-0.185, P=0.018) and positively with urinary albumin excretion (r=0.253, P=0.001). This indicated that increased blood viscosity is associated with reduced renal function and worsening of albuminuria in EHT. Stepwise multiple regression analysis identified blood viscosity as an independent determinant of creatinine clearance (R(2)=0.058) and urinary albumin excretion (R(2)=0.216). In conclusion, increased blood viscosity may be a risk for development of renal disease in EHT.