The efficacy and acceptability of the angiotensin-converting enzyme (ACE) inhibitor perindopril were compared with those of an established treatment for hypertension in Canada, slow release diltiazem. Hypertensive patients aged 18 to 64 years presenting a mean sitting diastolic blood pressure (DBP) between 95 and 110 mmHg inclusive were randomized if they were without any concurrent serious renal, hepatic, cardiac or psychiatric illnesses. A total of 49 of 83 patients qualified for randomization after four weeks on placebo; 23 were assigned to the perindopril treatment group and 26 to the diltiazem treatment group. Starting doses (4 mg od in the perindopril group and 120 mg [60 mg bid] in the diltiazem group) were increased to a maximum of 8 mg od in the perindopril group and to 360 mg (180 mg bid) in the diltiazem group if DBP remained above 90 mmHg. Hydrochlorothiazide was added at week 8 if goal blood pressure was not achieved with the maximum dose. Both groups were comparable at randomization; baseline systolic blood pressure (SBP)/DBP were 153/100 mmHg and 150/99 mmHg in the perindopril and diltiazem groups, respectively. A significant decrease in SBP/DBP was observed from the second week of treatment: -10/-8 mmHg in the perindopril group and -8/-8 mmHg in the diltiazem group. This fall in blood pressure was maintained throughout the duration of the trial: -13/-9 and -13/-11 mmHg for the perindopril and diltiazem groups, respectively, at week 12, at which time monotherapy was taken by over 80% of patients and compliance was consistently over 90%.(ABSTRACT TRUNCATED AT 250 WORDS)