BACKGROUND We analyzed the available data to assess the benefits of antihypertensive therapy in hypertensive patients with diabetes mellitus. METHODS A MEDLINE search of English-language articles published until June 1999 was undertaken using the terms diabetes mellitus, hypertension or blood pressure (BP), and therapy. Included were only prospective randomized studies of more than 12 months' duration that evaluated the effect of drug treatment on morbidity and mortality in diabetic hypertensive patients. RESULTS The coexistence of diabetes mellitus doubled the risk of cardiovascular events, cardiovascular mortality, and total mortality in hypertensive patients (approximate relative risk of 1.73 to 2.77 for cardiovascular events, 2.25 to 3.66 for cardiovascular mortality, and 1.73 to 2.18 for total mortality). Intensive BP control to levels lower than 130/85 mm Hg was beneficial in diabetic hypertensive patients. All four drug classes--diuretics, beta-blockers, angiotensin converting enzyme inhibitors, and calcium antagonists--were effective in reducing cardiovascular events in diabetic hypertensive patients. In elderly diabetic patients with isolated systolic hypertension, calcium antagonists reduced the rate of cardiac end points by 63%, stroke by 73%, and total mortality by 55%. In more than 60% of diabetic hypertensive patients, combination therapy was required to control BP. CONCLUSIONS Intensive control of BP reduced cardiovascular morbidity and mortality in diabetic patients regardless of whether low-dose diuretics, beta-blockers, angiotensin converting enzyme inhibitors, or calcium antagonists were used as a first-line treatment. Combination of more than one drug is frequently required to control BP and may be more beneficial than monotherapy.