Diabetic nephropathy has become the single most important cause of end-stage renal disease in the USA, Europe and Japan. The earliest marker of incipient diabetic nephropathy is the transition of normoalbuminuria to microalbuminuria at an albumin excretion rate of 20 microg/min. Human studies in patients both with and without diabetic kidney diseases have shown that the severity of baseline proteinuria is an important predictor of the rate of loss of renal function. Moreover, the reduction in protein excretion rate when patients with nephropathies are being treated with antihypertensive agents predicts the efficacy of subsequent renoprotection. Experimental and clinical observations provide the rationale for targeting the renin-angiotensin system as a renoprotective approach in diabetic and nondiabetic proteinuric nephropathies. Losartan (Cozaar, Merck Sharpe and Dohme) is a potent, orally active and highly specific angiotensin-type 1 receptor blocker. In addition to its antihypertensive efficacy, losartan decreases the left ventricular mass index in patients with hypertension, left ventricular end-diastolic and end-systolic volume in subjects with heart failure and prevents cardiovascular morbidity and death, predominantly stroke, independent of blood pressure reduction. Short-term studies in Type 1 diabetic patients with overt nephropathy have demonstrated that losartan and angiotensin-converting enzyme inhibitors have similar beneficial effects on albumin excretion rate, blood pressure and renal hemodynamics. Losartan also lowered albumin excretion rate in microalbuminuric patients with Type 2 diabetes mellitus. Moreover, the large multicenter Reduction of End points in Noninsulin dependent diabetes mellitus with the Angiotensin II Antagonist Losartan (RENAAL) trial has shown that blockade of angiotensin-type 1 receptor with losartan is superior to conventional antihypertensive therapy in slowing the progression of overt Type 2 diabetic nephropathy. Together, data from clinical trials demonstrate the beneficial effect of angiotensin-type 1 receptor blockers, including losartan, in the primary and secondary prevention of renal disease progression in diabetic patients. Nevertheless, it can be expected that the positive results achieved so far with this class of drugs may be further implemented by including angiotensin-type 1 receptor antagonists as a part of the multidrug approach that may hold more promise for the future of renoprotection in diabetic patients with chronic nephropathy.