The purpose of this study was to determine whether patients whose blood pressure failed to normalize while receiving monotherapy with atenolol would experience further blood pressure lowering by adding the angiotensin-converting enzyme (ACE) inhibitor benazepril hydrochloride to their treatment regimen. Seventy-four of the original 127 patients treated with atenolol met the criteria for entry into the 4-week, double-blind phase of the study, in which either benazepril 10 mg twice daily (increased after 1 week, if necessary, to 20 mg twice daily) or placebo was added to atenolol. At end point, 46% of the benazepril group had achieved an excellent or good response (ie, diastolic blood pressure [DBP] < 90 mm Hg or a decrease of > or = 10 mm Hg below the baseline) compared with 14% of the placebo group (P < 0.01). The mean fall in DBP at end point was -5.6 mm Hg in the benazepril group and -3.7 mm Hg in the placebo group. Because six patients in the benazepril group experienced an increase of blood pressure that offset the fall observed in the responders, the difference in DBP response between the benazepril group and the placebo group was not statistically significant. We conclude that adding benazepril to the regimen of patients whose blood pressure is inadequately controlled while receiving atenolol monotherapy can produce an additional decrease in blood pressure in almost half the patients.