Both day and night blood pressure have considerable ranges in normal individuals and also in diabetic patients. In addition, there is considerable variation intra-individually, with considerable excurses in blood pressure, e.g. during exercise, other daily activities as well as on exposure to medical personnel. There is good evidence to suggest that elevated blood pressure is an important factor in the progression of renal disease in diabetes, even from the initial phase of the slight elevation of the albumin excretion rate. From the earliest phase of microalbuminuria, blood pressure may increase by an average of 3-4 mmHg per year in contrast to 1 mmHg per year in healthy controls and in clearly normoalbuminuric individuals. Throughout the course of the complications of diabetes, both insulin-dependent and non-insulin-dependent, there is a correlation between albuminuria and blood pressure in cross-sectional studies; also there is a significant correlation between blood pressure and the progression of albuminuria. The same findings are available in essential hypertension and also to some extent in the background population, although in the latter the correlation between albuminuria and blood pressure is much less precise, although highly significant. Several trials conducted over the years uniformly show that antihypertensive treatment reduces albuminuria and, in many studies, progression in renal disease also, as measured by the glomerular filtration rate (GFR) fall. Therefore, it could be considered as a means to reduce blood pressure generally in diabetic individuals, even from the start of diabetes, with the aim of future further prevention of renal complications and possibly other complications. Such a proposal is less attractive in the background population because renal disease is much more rare. Another similar approach would be the prevention of renal disease, e.g. diabetics. Obviously, abnormalities in the vascular wall of a biochemical/functional nature may make diabetics more pressure-sensitive, and the indication is that several other risk factors are involved, in particular poor metabolic control. Nevertheless, it is proposed that trials should be conducted very early in the course of diabetes, to see if the same positive effect can be obtained early as that documented later in the course of microalbuminaria and overt renal disease, both in insulin-dependent and in non-insulin-dependent diabetes. In essential hypertension, antihypertensive treatment has a profound effect on albuminuria, and this may be associated with long-term renoprotection, but this is less well documented.