The association of arterial hypertension and diabetes mellitus is frequent: one third of patients attending a diabetic clinic. Excess hypertension frequency is marked in type II, non insulin-dependent diabetes, a condition often associated with other vascular risk factors such as obesity and lipid disorders. Insulin resistance is a common feature between type II diabetes, hypertension and other risk factors. In type I, insulin-dependent diabetes, hypertension is often linked to diabetic nephropathy. There is a genetic basis for diabetic nephropathy, which may share a common background with familial hypertension. Apart from possible genetic predispositions to hypertension diabetes association, chronic hyperglycaemia can lead to alteration in functional and structural properties of blood and vessels, which both contribute to elevated vascular resistance and blood pressure. From a therapeutic viewpoint, blood pressure values above 140/90 mmHg are not tolerable in diabetic subjects under 40 years of age. Due to their renal haemodynamic effects, angiotensin I converting enzyme inhibitors may be of special interest to protect kidney function in diabetic subjects.