Objective To study the effect of severe obesity on blood viscosity. Methods Blood viscosity and its major determinants were checked in 27 children with severe obesity and in 20 normal children. Results Plasma and whole blood viscosity were significantly increased in the obese group. The increase in low shear blood viscosity and aggregation index of RBC suggested an increased erythrocyte aggregation which can be explained mainly in terms of the effects of increased fibrinogen concentration and plasma viscosity, and the increase in high shear blood viscosity and TK (Deformability Index of RBC) indicated a decreased erythrocyte deformability which was associated with the levels of fibrinogen, cholesterol and plasma viscosity. Conclusions Severe obesity can cause blood hypercoagulation leading to a risk factor of cardiovascular and cerebrovascular diseases.