Apart from near normal metabolic control, early treatment of an increase in blood pressure in diabetic patients with nephropathy, is one of the most important therapeutic methods to prevent further progression of this complication. Long-term studies, recently published, suggest that ACE inhibitors have a beneficial effect on albuminuria and progression of nephropathy, irrespective of their hemodynamic effects. However, the mechanism by which ACE inhibitors exert these positive effects on glomerular pathology is still unclear. Several non-hemodynamic factors have been identified as being involved in the pathogenesis of diabetic nephropathy: (a) changes in the composition of glomerular basement membrane due to a changed metabolism of the proteins which make up this structure; consequences are an impairment of the filtration properties, onset of proteinuria as well as thickening of basement membrane; (b) Mesangial expansion due to an overproduction of mesangial matrix and deposition of proteins as well as (c) impairment of mesangial clearance function; consequences are development of glomerulosclerosis and reduction of filtration surface. It is known that the renin-angiotensin-system is stimulated in diabetic patients with nephropathy and that angiotensin II influences the synthesis of glomerular and mesangial proteins as well as the function of mesangial cells. Hypothetically, these points could explain the beneficial effects of ACE-inhibitors on the progression of diabetic nephropathy.