The clinical impression of an association between hypertension and diabetes mellitus has not been consistently confirmed by epidemiological studies. The variety of methodological problems in this area of research includes the choice of appropriate patient and control groups, the measurement techniques used, and consideration of many possible confounding variables. Several prospective studies have shown that hypertension may be a potent risk factor for diabetic macrovascular and microvascular disease. This relationship may have important preventive implications, and raises the question of the optimal antihypertensive therapy in diabetic patients.