In 1972--1973, 785 symptom-free men, aged 40 to 49 years, without target organ damage, with systolic blood pressures between 150 and 179 mm Hg and diastolic blood pressure below 110 mm Hg, were assigned at random to one of two groups: (406 to a drug treatment group and 379 to a control group) for a five-year controlled drug treatment trial to evaluate the effect of therapy on cardiovascular complications. Drug treatment started with hydrochlorothiazide. If systolic blood pressure remained above 140 mm Hg and/or diastolic blood pressure above 90 mm Hg, alphamethyldopa was added. If there were side effects, methyldopa was replaced with propranolol. The control group was not given a placebo. The mean observation time was 66 months (range 60 to 78 months). A difference in blood pressure between groups of about 17 mm Hg systolic and 10 mm Hg diastolic was maintained throughout the study. The study protocol had a rather low "ethical" blood pressure roof, 180 mm Hg systolic and/or 110 mm Hg diastolic. Seventeen percent of the control group had an increase in blood pressure above this level during the trial, and drug treatment was started. There was no effect on major cardiovascular morbidity comparing groups as established by randomization, with 18 events in the treatment group and 20 events in the control group. There was no difference between the groups in total mortality and mortality from cardiovascular events. However, in the subgroups with diastolic blood pressure greater than or equal to 100 mm Hg before randomization, there was a probable reduction in total morbidity from cardiovascular events in favor or the group receiving drug therapy, 7.6 and 16.4 percent events in the treated and control groups, respectively. Cerebrovascular events occurred only in the control group, 7 versus 0. Two cases of fatal aortic aneurysms also occurred in the control group. Other "pressure" complications, such as marked left ventricular hypertrophy in the electrocardiogram and left ventricular failure, occurred only in the control group. However, regarding coronary heart disease, including sudden death, the incidence tended to be higher in the treated group, although it was not statistically significant. Only 13 men (1.7 percent) failed to meet for regular examinations. At the end of the study these men were also followed up with regard to possible cardiovascular events.