The antihypertensive effects of drugs are partly determined by characteristics of the patients treated. A randomized, double-blind study used 24-hour ambulatory blood pressure (BP) monitoring to compare the effects of 2 beta blockers, bisoprolol (10 to 20 mg; n = 107) and atenolol (50 to 100 mg; n = 96), administered once daily in 4 population groups. After a 4-week placebo period, patients with an office diastolic BP between 95 and 114 mm Hg were stratified according to race and age, and were randomly assigned to treatment with bisoprolol or atenolol for 8 weeks. BP averages measured by automated monitoring for the 24-hour periods were compared between groups. In elderly patients, the reductions in both average 24-hour systolic and diastolic BP were greater with bisoprolol than with atenolol (13 +/- 3/13 +/- 1 mm Hg [n = 23] vs 4 +/- 2/6 +/- 1 mm Hg [n = 30]; p < 0.01). Similarly, bisoprolol produced greater reductions in average 24-hour diastolic BP than did atenolol in nonblack patients (16 +/- 2/12 +/- 1 mm Hg [n = 85] vs 12 +/- 2/9 +/- 1 mm Hg [n = 83]; p = 0.02). Bisoprolol and atenolol were similar in the black (10 +/- 5/9 +/- 3 mm Hg [n = 22] and 10 +/- 6/6 +/- 3 mm Hg [n = 13], respectively) and young (15 +/- 1/11 +/- 1 mm Hg [n = 84] and 16 +/- 2/10 +/- 1 mm Hg [n = 66], respectively) groups.(ABSTRACT TRUNCATED AT 250 WORDS)