Raised blood pressure is the strongest single risk factor for stroke in the general population. Diabetics are at increased risk of both hypertension and stroke. It is not clear if diabetes mellitus confers an excess risk of stroke that is independent of blood pressure. The authors examined the relation of diabetic status (personal history of diabetes and/or fasting plasma glucose greater than 7.8 mmol per liter) to stroke risk in a population-based cohort of 3,778 adults aged 50-79 years in Rancho Bernardo, California who were followed from 1972 for an average of 12 years. There were 232 stroke cases, 139 of which were ascertained from death certificates. Diabetics had higher mean systolic blood pressures, significantly so in females, and diabetics of both sexes were significantly more obese. Diabetics had greater univariate age-adjusted stroke mortality and morbidity rates than nondiabetics. The increased stroke rates were still apparent in diabetics after stratifying for systolic blood pressure. In multivariate analyses, the relative risks (RRs) for stroke mortality and morbidity associated with diabetes were not significantly changed in men (RR = 1.8) and women (RR = 2.2), after adjusting for the effect of risk factors including age, systolic blood pressure, cholesterol level, obesity, and smoking habits, and excluding persons with personal history of heart attack, heart failure, or stroke. These findings support the hypothesis that diabetes may confer excess risk of stroke independent of blood pressure.