Men aged 40-64 years with mild to moderate hypertension [diastolic blood pressure (DBP) 100-130 mmHg] were randomized to treatment with a diuretic (n = 3272) or a beta-blocker (n = 3297), with additional drugs if necessary, to determine whether a beta-blocker based treatment differs from thiazide diuretic based treatment with regard to the prevention of coronary heart disease (CHD) events and death. Patients with previous CHD, stroke or other serious diseases, or with contraindications to diuretics or beta-blockers were excluded. If normotension (DBP less than 95 mmHg) was not achieved by monotherapy, other antihypertensive drugs were added, but the two basic drugs were not crossed over. Patients were assessed at 6-monthly intervals. The mean follow-up for end-points was 45.1 months. Blood pressure (BP) side effects and end-points were recorded in a standardized manner. Entry characteristics and the BP reduction achieved were very similar in both treatment groups. All analyses were made on an intention-to-treat basis. The incidence of CHD did not differ between the two treatment groups. The incidence of fatal stroke tended to be lower in the beta-blocker treated group than in the diuretic treated group. Total mortality and the total number of end-points were similar in both groups. The percentage of patients withdrawn due to side effects was similar, whereas the number of reported symptoms, according to a questionnaire, was higher for patients on beta-blockers. The incidence of diabetes did not differ between the two groups. Subgroup analyses did not detect a difference in the effect of beta-blockers compared with diuretics in smokers as opposed to non-smokers, and beta-blockers also had the same effects as diuretics in the quartile with the highest predicted risk for CHD. Beta-blockers and thiazide diuretics were approximately equally well tolerated. The two drugs had a similar BP reducing effect although additional drugs had to be given more often in the diuretic group. Antihypertensive treatment based on a beta-blocker or on a thiazide diuretic could not be shown to affect the prevention of hypertensive complications, including CHD, to a different extent.