To determine the prevalence and control of hypertension in the adult population of Rochester, Minnesota, a blood pressure survey was conducted among a stratified random sample of 2,122 subjects 35 years of age or older. Hypertension--defined as a history of hypertension or a systolic pressure of 160 mm Hg or more, a diastolic pressure of 95 mm Hg or more, or both at the time of interview--was found in 663 subjects (31%). Among these hypertensive subjects, 82 (12%) had no prior history of hypertension. Of the 581 subjects with a history of hypertension, 447 (77%) were taking antihypertensive medication, 127 (22%) had a systolic pressure of 160 mm Hg or more, a diastolic pressure of 95 mm Hg or more, or both, and 288 (50%) had a systolic pressure of 140 mm Hg or more, a diastolic pressure of 90 mm Hg or more, or both. Of the treated patients with hypertension, 75% were taking a diuretic either alone or in combination with one or more other agents. Side effects of therapy were reported by 12% of subjects taking antihypertensive medication.