The Hypertension Optimal Treatment (HOT) Study is a prospective, randomized, multicenter trial being conducted in 26 countries. Its main aim is to evaluate the relationship between three levels of target diastolic blood pressure (< or = 90, < or = 85 or < or = 80 mmHg) and cardiovascular morbidity and mortality in hypertensive patients. In addition, the study will examine the effects on morbidity and mortality of a low dose, 75 mg daily, of acetylsalicylic acid (ASA, aspirin) or placebo. In the HOT Study, basic antihypertensive treatment is initiated with the calcium antagonist felodipine at a dose of 5 mg daily. If target blood pressure is not reached, additional antihypertensive therapy with either an angiotensin converting enzyme (ACE) inhibitor or a beta-adrenoceptor blocking agent is given. Further dosage adjustments are made in accordance with a set protocol. As a fifth and final step, a diuretic may be added. Inclusion of patients was stopped on April 30, 1994. At that time 19,196 patients had been randomized. There were 9,055 (47%) women and 10,141 (53%) men with an average age of 61.5 +/- 7.5 (SD) years. At enrollment, 52% of patients were receiving antihypertensive treatment. These patients entered a wash-out period of at least 2 weeks before randomization. The average randomization blood pressure in untreated patients was 169 +/- 14/106 +/- 3 mmHg and in the treated patients 170 +/- 14/105 +/- 3 mmHg. On August 15, 1994, blood pressure data were available for 14,710 and 10,275 patients, who had completed 3 and 6 months treatment, respectively. The average reduction in diastolic blood pressure was 22 mmHg after 6 months.(ABSTRACT TRUNCATED AT 250 WORDS)