Hypertension in type I diabetes appears to be causally related to nephropathy, whereas the relationship between hypertension and type II diabetes is more complex, hypertension being present in the absence of clinically overt nephropathy and even preceding the onset of diabetes. In the diabetic, hypertension is exquisitely sodium-sensitive. It is in diabetic nephropathy that the best evidence has been obtained that treatment of hypertension retards the progression of renal failure. Consensus is still lacking with respect to the point when antihypertensive treatment should be started and the target blood pressure that should be aimed at. Currently, there is no evidence in humans that converting enzyme inhibitors are superior to alternative antihypertensive agents in retarding progression, but tantalizing preliminary evidence on this has been reported in nondiabetic patients with renal failure.