Smoking is a universally accepted major cardiovascular risk factor, but the mechanisms by which it promotes ischaemic vascular disease are not fully understood. The changes that chronic smoking exerts on the flow properties of blood might contribute to an explanation. It is well documented that smoking leads to a rise in haematocrit. It also alters the rheological behaviour of red blood cells and increases both plasma viscosity and fibrinogen levels. Finally, it increases the total white cell count and modifies leukocyte function. Together these changes cumulate in a significant deterioration of the flow properties of blood, as evidenced by a steep increase in whole blood viscosity. Alterations of blood rheology in turn can promote atherothrombogenesis in several ways. It seems possible, therefore, that one mechanism by which smoking increases the risk of vascular diseases operates through its complex effects on blood rheology.