Blood viscosity studies were carried out in fourteen patients with acute stroke, eight with cerebral infarction, six with cerebral haemorrhage and in thirteen controls. We observed a statistically significant higher values of plasma, red cell and whole viscosity in patients with acute stroke than in normal controls. Plasma fibrinogen levels were statistically higher (p less than 0.01) in patients than in normal controls. The platelet aggregation was increased in two young adults with acute stroke. The results suggest that the haemorheological factors play an important role in the pathophysiology of stroke patients.