In order to evaluate slimming effects on certain metabolic (cholesterol, triglycerides, basal insulinaemia) and haemorheologic (haematocrit, fibrinogen, whole blood viscosity and plasma viscosity) rates, we studied 24 obese subjects (15 female and nine male) aged 25-58, with BMI ranging from 35.5 to 67, before and after a hypocaloric diet period involving a 20-kg weight loss. All subjects underwent blood sampling to assess: OGTT, plasma proteins, serum total cholesterol, HDL-cholesterol, serum triglycerides, haematocrit, whole blood viscosity at high and low shear rate with and without correction of 45 per cent haematocrit, plasmatic capillary viscosity and fibrinogen level. Our study showed no significant changes in plasma proteins, serum HDL-cholesterol, haematocrit, fibrinogen and whole blood viscosity at high and low shear rate, while basal insulinaemia, total cholesterol, triglycerides, low shear rate corrected at 45 per cent haematocrit blood viscosity and capillary plasmatic viscosity are significantly reduced. However the analysis of possible relation between the differences of assessed rates before and after slimming has shown no significant correlations. In conclusion, we can say that the slimming of very obese subjects improves blood and plasma viscosity, but the mechanism by which this improvement occurs is not the one which usually affects the determination of these rates.