The BBB study is a multicentre trial designed to investigate three aspects of antihypertensive treatment of great clinical importance: (1) Is it possible to lower diastolic blood pressure further in treated hypertensive patients (to ≤80 mmHg) through intensified therapeutic activities? (2) Can this aim be fulfilled without increasing the incidence or severity of side effects to unacceptable levels? (3) If 1 and 2 are answered in the affirmative, will the reduction in diastolic blood pressure give a further reduction in hypertension-associated mortality and morbidity? The BBB study is a prospective trial in which male and female treated hypertensive patients, aged 45–67 years, with treated diastolic blood pressures in the range 90–100 mmHg, will be randomly allocated to either unchanged antihypertensive treatment or intensified treatment with the aim of lowering the diastolic blood pressure to ≤80 mmHg. Statistical calculations indicate that approximately 2000 patients studied for 3 years will be needed to answer the third question. For this reason the trial will be conducted as a multicentre study performed mainly by primary health care physicians.