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Objective: Alterations of blood and plasma viscosity can promote atherosclerosis. The relationship between viscosity and aging is still controversial. The present study evaluated the influence of aging on blood and plasma viscosity in a group of subjects followed for 11.6 years. Methods: Forty-five subjects have been evaluated twice 11.6 years apart for hemorheological parameters and coronary heart disease (CHD) risk factors. Plasma viscosity and blood viscosity have been measured with a cone-plate viscometer. Tk has been calculated as index of red blood cell rigidity. CHD risk factors, i.e. obesity, hypertension, hyperlipidemia and diabetes, have been evaluated by routine methods. Results: Hematocrit and plasma viscosity did not change during the study, whereas blood viscosity (shear rate 225/sec: 4.46 ± 0.49 vs. 4.81 ± 0.54 cP, p < 0.0001; shear rate 45/sec: 6.19 ± 0.67 vs. 6.65 ± 0.79 cP, p < 0.0001) and Tk (0.80 ± 0.05 vs. 0.83 ± 0.06, p < 0.005) significantly increased. The percent variation in blood viscosity was not associated with the percent variation in any of the CHD risk factors. Furthermore, the increase in blood viscosity was similar in males and females and in subjects with CHD risk profile worsening or not. Conclusion: The present findings demonstrate that blood viscosity increases with age. This increase seems independent of classical CHD risk factors and is disjoined from haematocrit and plasma viscosity, suggesting a possible direct effect of aging on red blood cells. [ABSTRACT FROM AUTHOR]