The efficacy and safety of carvedilol, a beta-blocker with vasodilating properties, were compared at a dosage of 25 to 50 mg once daily with those of atenolol at a dosage of 50-100 mg once daily in a double-blind, randomized, parallel-group, multicenter study. After a single-blind placebo phase of 3 to 6 weeks, 47 patients (median age, 59 years) were randomized to receive carvedilol and 52 patients (median age, 57 years) were randomized to receive atenolol for an 8-week study period. Patients on carvedilol received 12.5 mg for the first 2 days and then 25 mg as a once-daily dosage. The initial dosage of atenolol was 50 mg once daily. The dosage of each treatment could be doubled (to 50 and 100 mg once daily, respectively) at week 4 if the response was inadequate. Sitting and standing blood pressures and heart rates were recorded 24 h after the dose at weeks 4 and 8. Data from 90 of 98 patients who completed the study were eligible for per-protocol analysis. Approximately one-third of the patients in each group required upward dose titration at week 4 because of inadequate response. At week 8, 84% patients receiving carvedilol and 91% receiving atenolol had sitting diastolic blood pressure less-than-or-equal-to 90 mm Hg or decreased their blood pressure by greater-than-or-equal-to 10 mm Hg (95% confidence intervals for difference between carvedilol and atenolol, +7% and -21%). Safety profiles were similar between treatments. One patient withdrew; a skin rash developed during the fourth week of treatment with atenolol. There were no apparent clinically relevant trends in abnormal laboratory parameters. At the doses compared in this study, carvedilol and atenolol are similar in efficacy and tolerability.