We compared the safety and efficacy of diltiazem and propranolol, and examined demographic factors influencing responses to these agents. One hundred ninety-six patients with supine diastolic blood pressures of 95 to 114 mm Hg were treated with propranolol (80 to 240 mg twice a day) or a sustained-release preparation of diltiazem (60 to 180 mg twice a day) in a double-blind, randomized, parallel group protocol for 6 months. Hydrochlorothiazide was added for patients not achieving the treatment goal. Both agents produced nearly identical and highly significant (p less than 0.001) reductions in supine blood pressure. There were no significant differences at the end of the optional combination therapy phase, although additional reduction with hydrochlorothiazide was slightly greater in the propranolol group. Blood pressure responses in relation to age, gender, race, and smoking history showed that diltiazem produced greater changes in older subjects and women, whereas propranolol was less effective in blacks. However, these differences were not critical.