The benefit of pharmacological treatment of hypertension in the prevention of cardiovascular diseases has been established mainly in subjects younger than 80 years of age. The consequences of undertreatment of hypertension with regard to the occurrence of stroke was studied in the Netherlands. About 33% of the hypertensives younger than 80 years and 55% of the hypertensives older than 80 years were not pharmacologically treated, although they were 'candidates' for pharmacological treatment of hypertension. About 19% (n = 1350) of all strokes that occurred in the Netherlands in 1994 among hypertensives younger than 80 years may be attributable to untreated hypertension. Among hypertensives older than 80 years this percentage was 32% (n = 1345). Although the benefit of pharmacological treatment of hypertension in persons older than 80 years has not been established experimentally, the results suggest that a considerable proportion of the strokes among this group of hypertensives may be prevented.