Patients who had suffered an ischaemic stroke were randomized to receive either oral pyritinol or placebo for 4 weeks. Blood and plasma viscosity, haematocrit and red cell filterability were quantified at baseline as well as 1 and 4 weeks after. No changes were seen in the placebo group. Blood viscosity decreased significantly in the experimental group. Since this variable can be viewed as a secondary risk factor for stroke survivors, one should test the possibility that pyritinol may be useful in the secondary prevention after stroke.