No adverse effect upon cognitive function has been detected during the first 9 months of treatment by a thiazide diuretic (Moduretic; Merck Sharp & Dohme, Hoddesdon, UK) or a beta-blocking agent (atenolol) of moderately elevated blood pressure in a cohort of 2630 men and women aged between 65 and 74 years. Furthermore, no association was discovered between levels of systolic or diastolic blood pressure at outset and responses to standard tests of cognitive function. This study was carried out in general practices as part of the current Medical Research Council Treatment Trial of Mild Hypertension in the Elderly.