To determine the antihypertensive efficacy of aerobic exercise training in mild essential hypertension, a prospective randomized controlled trial was conducted comparing an aerobic exercise regimen to a placebo exercise regimen, with a crossover replication of the aerobic regimen in the placebo exercise group. The study took place in an outpatient research clinic in a university-affiliated Veterans Administration medical center. Twenty-seven men with untreated diastolic blood pressure (DBP) of 90-104 mm Hg were randomized to the two exercise regimens. Ten patients completed the aerobic regimen. Nine patients completed the control regimen, seven of whom subsequently entered and completed the aerobic regimen. The aerobic regimen consisted of walking, jogging, stationary bicycling, or any combination of these activities for 30 minutes, four times a week, at 65-80% maximal heart rate. The control regimen consisted of slow calisthenics and stretching for the same duration and frequency but maintaining less than 60% maximal heart rate. DBP decreased 9.6 +/- 4.7 mm Hg in the aerobic exercise group but increased 0.8 +/- 6.2 mm Hg in the placebo control exercise group (p = 0.02). Systolic blood pressure (SBP) decreased 6.4 +/- 9.1 mm Hg in the aerobic group and increased 0.9 +/- 9.7 mm Hg in the control group (p = 0.11). Subsequently, seven of the nine controls entered a treatment crossover and completed the aerobic regimen with significant reductions in both DBP (-6.1 +/- 3.2 mm Hg, p less than 0.01) and SBP (-8.1 +/- 5.7 mm Hg, p less than 0.01). BP changes were not associated with any significant changes in weight, body fat, urinary electrolytes, or resting heart rate. This randomized controlled trial provides evidence for the independent BP lowering effect of aerobic exercise in unmedicated mildly hypertensive men.