Interest in evidence-based medicine is increasing greatly, with the focus on treatment that prevents organ failure and that may prolong life. Type 1 and Type 2 diabetes are conditions associated with increased mortality, mainly as a result of renal and cardiovascular diseases, and blindness. All three complications usually occur together. In recent years, more focus has been placed on treating patients early to prevent future organ damage. Microalbuminuria is an important intermediary end-point that correlates strongly with future advanced renal disease, retinopathy and mortality. Several trials have studied patients with microalbuminuria and also patients in more advanced stages of the disease who have proteinuria (termed overt nephropathy). Recent evidence indicates that achieving optimal glycaemic control reduces the risk of an increase in urinary albumin excretion before the development of microalbuminuria. Angiotensin-converting enzyme (ACE) inhibitors are effective in reducing microalbuminuria, partly independent of their blood pressure reducing effects. In Type 1 and Type 2 diabetic patients with microalbuminuria, long-term treatment with ACE inhibitors (7-8 years) prevents the predicted decrease in glomerular filtration rate (GFR); optimal glycaemic control is also important in preventing the decline in GFR. This is important because GFR is usually well preserved in Type 1 and Type 2 diabetic patients with microalbuminuria and a predicted decline in GFR can therefore be prevented. In overt renal disease, studies that focused mostly on Type 1 diabetic patients have shown that the rate of decline in GFR can be reduced. Long-term studies in Type 1 diabetic patients have also demonstrated that mortality caused by end-stage renal disease can be postponed. Mortality associated with cardiovascular diseases, e.g. myocardial infarction, is reduced more effectively in diabetic patients treated with ACE inhibitors and beta-blockers than in non-diabetic patients treated with the same drugs. Screening for microalbuminuria, the attainment of optimal glycaemic control, and early treatment with ACE inhibitors and other antihypertensive drugs are necessary to prevent progression of diabetic complications, especially diabetic nephropathy. However, there is some controversy about the initial use of calcium channel blockers. In conclusion, early achievement of improved glycaemic control is the most important factor in the prevention of diabetic complications. Antihypertensive treatment is clearly also important.