The Hypertension Optimal Treatment (HOT) Study is an ongoing prospective randomized, multicentre trial conducted in 26 countries. There are two main aims of the study. The first is to evaluate the relationship between three levels of target diastolic blood pressure (< or = 90, < or = 85 or < or = 80 mmHg) and the incidence of cardiovascular morbidity and mortality in hypertensive patients. The second is to determine the effect on morbidity and mortality of a low dose, 75 mg daily, of acetylsalicylic acid (ASA, aspirin) compared with placebo. Altogether 18,790 patients have been recruited and randomized, and two-year data are now available for all patients. This is a report on the blood pressures achieved, the tolerability, and other available data after 24 months of follow-up of all patients. Special emphasis is given to the subgroup of elderly patients (> or = 65 years, n = 5988) compared with young patients (< 65 years, n = 12 802). On average, patients in the < or = 90 mmHg diastolic blood pressure target group have reached 85 mmHg, in the < or = 85 mmHg target group patients have reached 83 mmHg and in the < or = 80 mmHg target group patients have reached 81 mmHg. The percentage of those achieving target blood pressure in each target group at 24 months of follow-up is 85% in the < or = 90 mmHg target group, 75% in the < or = 85 mmHg target group and 57% in the < or = 80 mmHg target group. In the elderly subgroup (> or = 65 years of age), the percentage of patients achieving target at 24 months is higher for all target groups, namely 89% in the < or = 90 mmHg group, 80% in the 85 mmHg group and 62% in the 80 mmHg group. Antihypertensive treatment was initiated with a calcium antagonist, felodipine, at a dose of 5 mg once daily. If target blood pressure was not reached, additional antihypertensive therapy, with either an angiotensin converting enzyme (ACE) inhibitor or a beta-adrenoceptor blocking agent, was given. Further dose adjustments were made in accordance with a set protocol. As a fifth, and final, step, a diuretic could be added. There have been relatively few side effects in this large, multinational study of hypertensive patients. Only ankle oedema and coughing exceed a frequency of 0.5% (ankle oedema 1.3% in young and 1.7% in elderly; coughing 0.5% in young and elderly). After two years, 84% of all patients are still taking their baseline therapy, felodipine. The 24-month data presented here indicate that it should be possible to fulfil the primary aims of the HOT Study.