The blood viscosity parameters of one hundred and fifty cases of WHO grade I and II hypertension of Indian origin, on treatment with calcium antagonists, were measured and a six-monthly follow up was conducted for a period of three years. The whole blood viscosity (WBV), plasma viscosity (PV), red cell rigidity (RG) and hematocrit (Hct) were monitored. Occurrence of stroke was considered as the outcome variable and it was observed that sixteen cases of stroke occurred during the follow up period (10.7% incidence). After excluding known risk factors of age, sex, addictions, blood pressure levels, cholesterol levels and underlying diseases, it was observed that an increased whole blood viscosity > 6 centiPoise was associated with an increased risk of stroke (Relative risk 2.9, 95% confidence interval 2.2 to 3.7). An increased red cell rigidity in a hypertensive patient was found to be an independent risk factor for stroke. Patients with red cell rigidity greater than 4 had 4 times (Relative risk 3.6, 95% confidence interval 3.2 to 4) higher risk of stroke as compared with patients with red cell rigidity levels less than 4. Treatment with drugs modifying the rheological profile and aiming at improving the red cell deformability should thus be considered in hypertensive patients in an attempt to prevent the occurrence of stroke.