To investigate the concept of initiating therapy with low doses of a calcium antagonist and an ACE inhibitor, a fixed combination of isradipine 2.5 mg plus the ACE inhibitor spirapril 3 mg was compared with its components, with the full-dose monotherapies (isradipine 5 mg or spirapril 6 mg), and with placebo. After a 2-week wash out phase in pretreated patients and a subsequent 2-week placebo period, 405 patients with a diastolic blood pressure (DBP) between 100 and 114 mmHg were randomly allocated to 12-week once-daily double-blind treatment in one of the six treatment arms. In patients whose blood pressure was not normalized (defined as DBP< or =90 mmHg) after 6 weeks of treatment, the dosage of either medication was doubled or, in the placebo group, was switched to the fixed combination. After week 6, the mean reductions from baseline in sitting systolic/diastolic blood pressure 24 hours after dosing (trough) for the fixed combination or the monotherapies isradipine 5 mg, isradipine 2.5 mg, spirapril 6 mg, spirapril 3 mg, and placebo were 10.4/8.7, 10.0/9.4, 6.5/6.7, 10.0/8.3, 7.0/5.8, and 2.2/4.7 mmHg, respectively. The blood pressure changes obtained with the low-dose fixed combination were essentially identical to those observed with the full-dose monotherapies, thus showing an additive effect of low-dose isradipine and spirapril. In terms of tolerability, the lowest rate of any adverse events was found in the combination group. In this group, typical adverse events of calcium antagonists, such as headache, flushing, ankle edema, or palpitations, were observed only in 5%, 2%, 1%, and 0%, respectively, dry cough, considered typical for ACE inhibitors, was observed in only 1% of the combination group. In conclusion, the low-dose components isradipine 2.5 mg and spirapril 3 mg were shown to have an additive effect when combined, exerting a blood pressure-lowering effect comparable with the full doses and a trend to a better tolerability profile in comparison with the standard doses. Thus, low-dose combination therapy with these drugs appears to be a rational alternative to conventional monotherapy in the first-line treatment of hypertension.