In this 12-week, double-blind, parallel-group, comparative trial, 57 adult patients with mild-to-moderate hypertension were randomly allocated to receive imidapril or captopril, initially at a dose of 5 mg once a day and 25 mg twice daily, respectively. After 4 weeks of therapy, the dose of each drug was increased twice if diastolic blood pressure (DBP) remained > or =90 mm Hg. Both treatments effectively lowered DBP in a comparable manner. Mean changes from baseline in DBP at 12 weeks were -9.9 mm Hg for imidapril and -8.8 mm Hg for captopril (p = 0.488). Responder rates in patients receiving active treatment for at least 6 weeks were 53.9% for imidapril and 48% for captopril (p = 0.676). Both treatments were well tolerated. Adverse drug reactions were observed in 20.7% (6/29) of the imidapril group and 46.4% (13/28) of the captopril group (p < 0.05). A cough was the most frequent side effect, reported in 13.8% of the imidapril group and 35.7% of the captopril group. The results indicate that imidapril is as effective as captopril in the treatment of hypertension. Imidapril produces less adverse effects compared with captopril.