Risk factors for atherosclerosis are often associated with haemorheological changes. On this point, obesity (recently advocated as an independent risk factor) was not much studied and with not univocal results. We have studied 70 obese patients (BMI greater than 30) and 50 healthy subjects (BMI less than 25). Among obese 26 had no more pathologies, 29 had hypertension, 3 suffered from ischemic heart disease, 3 suffered from occlusive arteriopathy, 9 were hyperlipidemic, 10 were smokers. We determined plasma viscosity and whole blood viscosity (at haematocrit corrected to 45% too). Washed erythrocytes, poor in leucocytes and platelets and resuspended in phosphate-buffered saline, were used for study of erythrocyte viscosity and deformability. Obese patients showed raised mean blood viscosity values when compared to healthy controls (p less than 0.01); an even more significant increase (p less than 0.001) was found concerning plasma viscosity and fibrinogen. Erythrocyte viscosity and red blood cell filterability index did not show any significant difference. We found no significant correlation between viscosity values and presence of hypertension, hyperlipidemia and smoking habit among obese. In conclusion, the higher vasculopathy incidence might be caused by an increase in blood viscosity, mostly due to plasmatic component. This fact appears to be independent from the presence of atherosclerosis complications or other risk factors.